1911 Encyclopædia Britannica/Mycetozoa

by Joseph Jackson Lister

MYCETOZOA (Myxomycetes, Schleimpilze), in zoology, a group of organisms reproducing themselves by spores. These are produced in or on sporangia which are formed in the air and the spores are distributed by the currents of air. They thus differ from other spore-bearing members of the animal kingdom (which produce their spores while immersed in water or, in the case of parasites, within the fluids of their hosts), and resemble the Fungi and many of the lower green plants. In relation with this condition of their fructification the structures formed at the spore-bearing stage to contain or support the spores present a remarkable resemblance to the sporangia of certain groups of Fungi, from which, however, the Mycetozoa are essentially different.

Although the sporangial and some other phases have long been known, and Fries had enumerated 192 species in 1829, the main features of their life-history were first worked out in 1859–1860 by de Bary (1 and 2). He showed that in the Mycetozoa the spore hatches out as a mass of naked protoplasm which almost immediately assumes a free-swimming flagellate form (zoospore), that after multiplying by division this passes into an amoeboid phase, and that from such amoebae the plasmodia arise, though the mode of their origin was not ascertained by him.

The plasmodium of the Mycetozoa is a mass of simple protoplasm, without a differentiated envelope and endowed with the power of active locomotion. It penetrates the interstices of decaying vegetable matter, or, in the case of the species Badhamia utricularis, spreads as a film on the surface of living fungi; it may grow almost indefinitely in size, attaining under favourable conditions several feet in extent. It constitutes the dominant phase of the life-history. From the plasmodium the sporangia take their origin. It was Cienkowski who (in 1863) contributed the important fact that the plasmodia arise by the fusion with one another of numbers of individuals in the amoeboid phase—a mode of origin which is now generally recognized as an essential feature in the conception of a plasmodium, whether as occurring among the Mycetozoa or in other groups (7). De Bary clearly expressed the view that the life-history of the Mycetozoa shows them to belong not to the vegetable but to the animal kingdom.

The individual sporangia of the Mycetozoa are, for the most part, minute structures, rarely attaining the size of a mustard-seed, though, in the composite form of aethalia, they may form cake-like masses an inch or more across (fig. 21). They are found, stalked or sessile, in small clusters or distributed by the thousand over a wide area many feet in diameter, on the bark of decaying trees, on dead leaves or sticks, in woods and shrubberies, among the stems of plants on wet moors, and, generally, at the surface in localities where there is a substratum of decaying vegetable matter sufficiently moist to allow the plasmodium to live. Tan-heaps have long been known as a favourite habitat of Fuligo septica, the plasmodia of which, emerging in bright yellow masses at the surface prior to the sporangial (in this case aethalial) phase, are known as “flowers of tan.” The film-like, expanded condition of the plasmodium, varying in colour in different species and traversed by a network of vein-like channels (fig. 5), has long been known. The plasmodial stage was at one time regarded as representing a distinct group of fungi, to which the generic name Mesenterica was applied. The species of Mycetozoa are widely distributed over the world in temperate and tropical latitudes where there is sufficient moisture for them to grow, and they must be regarded as not inconsiderable agents in the disintegrating processes of nature, by which complex organic substances are decomposed into simpler and more stable chemical groups.

Classification.—The Mycetozoa, as here understood, fall into three main divisions. The Endosporeae, in which the spores are contained within sporangia, form together with the Exosporeae, which bear their spores on the surface of sporophores, a natural group characterized by forming true plasmodia. They constitute the Euplasmodida. Standing apart from them is the small group of the mould-like Sorophora, in which the amoeboid individuals only come together immediately prior to spore-formation and do not completely fuse with one another.

A number of other organisms living on vegetable and animal bodies, alive or dead, and leading an entirely aquatic life, are included by Zopf (31) under the Mycetozoa, as the “Monadina,” in distinction from the “Eumycetozoa,” consisting of the three groups above mentioned. The alliance of some of these (e.g. Protomonas) with the Mycetozoa is probable, and was accepted by de Bary, but the relations of other Monadina are obscure, and appear to be at least as close with the Heliozoa (with which many have in fact been classed). The limits here adopted, following de Bary, include a group of organisms which, as shown by their life-history, belong to the animal stock, and yet alone among animals^[1] they have acquired the habit, widely found in the Vegetable kingdom, of developing and distributing their spores in air.

Class MYCETOZOA.
Sub-class 1.—Euplasmodida.[2]
Division 1.—Endosporeae.
Cohort 1.—Amaurosporales.
Sub-cohort 1.—Calcarineae.
Order 1.	Physaraceae. Genera: Badhamia, Physarum, Physarella, Trichamphora, Erionema, Cienkowskia, Fuligo, Craterium, Leocarpus, Chondrioderma, Diachaea.
Order 2.	Didymiaceae. Genera: Didymium, Spumaria, Lepidoderma.
Sub-cohort 2.—Amaurochaetineae.
Order 1.	Stemonitaceae. Genera: Stemonitis, Comatricha, Enerthenema, Echinostelium, Lamproderma, Clastoderma.
Order 2.	Amaurochaetaceae. Genera: Amaurochaete, Brefeldia.
Cohort 2.—Lamprosporales.
Sub-cohort 1.—Anemineae.
Order 1.	Heterodermaceae. Genera: Lindbladia, Cribraria, Dictydium.
Order 2.	Licaeceae. Genera: Licea, Orcadella.
Order 3.	Tubulinaceae. Genera: Tubulina, Siphoptychium, Alwisia.
Order 4.	Reticulariaceae. Genera: Dictydiaethalium, Enteridium, Reticularia.
Order 5.	Lycogalaceae. Genus: Lycogala.
Sub-cohort 2.—Calonemineae.
Order 1.	Trichiaceae. Genera: Trichia, Oligonema, Hemitrichia, Cornuvia.
Order 2.	Arcyriaceae. Genera: Arcyria, Lachnobolus, Perichaena.
Order 3.	Margaritaceae. Genera: Margarita, Dianema, Prototrichia, Listerella.
Division 2.—Exosporeae.
Order 1.	Ceratiomyxaceae. Genus: Ceratiomyxa.
Sub-class 2.—Sorophora.
Order 1.	Guttulinaceae. Genera: Copromyxa, Guttulina, Guttulinopsis.
Order 2.	Dictyosteliaceae. Genera: Dictyostelium, Acrasis, Polysphondylium.

LIFE-HISTORY OF THE MYCETOZOA

Euplasmodida

Endosporeae.

After A. Lister.

Fig. 1.^[3]—Stages in the Hatching of the Spores of Didymium difforme.

𝑎, The unruptured spore.

𝑏, The protoplasmic contents of the spore emerging. It contains a nucleus with the (light) nucleolus, and a contractile vacuole (shaded).

𝑐, The same, free from the spore wall.

𝑑, Zoospore, with nucleus at the base of the flagellum, and contractile vacuole.

𝑒, A zoospore with pseudopodial processes at the posterior end, to one of which a bacillus adheres. Two digestive vacuoles in the interior contain ingested bacilli.

𝑓, Amoeboid phase with retracted flagellum.

We may begin our survey of the life-history at the point where the spores, borne on currents of air, have settled among wet decaying vegetable matter. Shrunken when dry, they rapidly absorb water and resume the spherical shape which is found in nearly all species. Each is surrounded by a spore wall, sheltered by which the protoplasm, though losing moisture by drying, may remain alive for as many as four years. In several cases it has been found to give the chemical reaction of cellulose. It is smooth or variously sculptured according to the species. Within the protoplasm may be seen the nucleus, and one or more contractile vacuoles make their appearance. After the spore has lain in water for a period varying from a few hours to a day or two the wall bursts and the contained protoplasm slips out and lies free in the water as a minute colourless mass, presenting amoeboid movements (fig. 1, 𝑐). It soon assumes an elongated piriform shape, and a flagellum is developed at the narrow end, attaining a length equal to the rest of the body. The minute zoospore, thus equipped, swims away with a characteristic dancing motion. The protoplasm is granular within but hyaline externally (fig. 1, 𝑑). The nucleus, lying at the end of the body where it tapers into the flagellum, is limited by a definite wall and contains a nuclear network and a nucleolus. It often presents the appearance of being drawn out into a point towards the flagellum, and a bell-like structure [first described by Plenge (27)], staining more darkly than the rest of the protoplasm, extends from the base of the flagellum and invests the nucleus (fig. 2, 𝑎 and 𝑐). The other end of the zoospore may be evenly rounded (fig. 1, 𝑑) or it may be produced into short pseudopodia (fig. 1, 𝑒). By means of these the zoospore captures bacteria which are drawn into the body and enclosed in digestive vacuoles. A contractile vacuole is also present near the hind end. Considerable movement may be observed among the granules of the interior, and in the large zoospores of Amaurochaete atra this may amount to an actual streaming, though without the rhythm characteristic of the plasmodial stage.

Fig. 2.—Zoospores of Badhamia panicea, stained. In 𝑎 and 𝑐 the bell-like structure investing the nucleus is clearly seen.

After A. Lister. Fig. 3.—Three stages in the division of the Zoospore of Reticularia Lycoperdon.

Other shapes may be temporarily assumed by the zoospore. Attaching itself to an object it may become amoeboid, either with (fig. 1, 𝑓) or without (fig. 2, 𝑐) the temporary retraction of the flagellum; or it may take an elongated slug-like shape and creep with the flagellum extended in front, with tactile and apparently exploratory movements.

That the zoospores of many species of the Endosporeae feed on bacteria has been shown by A. Lister (18). New light has recently been thrown on the matter by Pinoy (26), who has worked chiefly with Sorophora, in which, as shown below, the active phase of the life-history is passed mainly in the state of isolated amoebae. Pinoy finds that the amoebae of this group live on particular species of bacteria, and that, the presence of the latter is a necessary condition for the development of the Sorophora, and even (as has been recognized by other workers) for the hatching of their spores. Pinoy's results indicate, though not so conclusively, that bacteria are likewise the essential food of the Euplasmodida in the early phases of their life-history. The zoospores do, however, ingest other solid bodies, e.g. carmine granules (Saville Kent, 15).

The zoospores multiply by binary fission the flagellum being withdrawn and the nucleus undergoing mitotic division, with the formation of a well-marked achromatic spindle (fig. 3).

It is probable that fission occurs more than once in the zoospore stage; but there is not satisfactory evidence to show how often it may be repeated.^[4]

After A. Lister.

Fig. 4.—Amoebulae of Didymium difforme uniting to form a Plasmodium. The common mass contains digestive vacuoles (v). The clear spherical bodies are microcysts and an empty spore-shell is seen to the left.

At this, as at other phases of the life-history, a resting stage may be assumed as the result of drying, but also from other and unknown causes. The flagellum is withdrawn and the protoplasm, becoming spherical, secretes a cyst wall. The organism thus passes into the condition of a microcyst, from which when dry it may be awakened to renewed activity by wetting.

At the end of the zoospore stage the organism finally withdraws its flagellum and assumes the amoeboid shape. It is now known as an amoebula. The amoebulae become endowed, as was first recognized by Cienkowski, with mutual attraction, and on meeting fuse with one another. Fig. 4 represents a group of such amoebulae. Several have already united to form a common mass, to which others, still free, are converging. The protoplasmic mass thus arising is the plasmodium. The fusion between the protoplasmic bodies of the amoebulae which unite to form it is complete. Their nuclei may be traced for some time in the young plasmodium and no fusion between them has been observed at this stage (20). As the plasmodium increases in size by the addition of amoebulae the task of following the fate of the individual nuclei by direct observation becomes impossible.

Fig. 5.—Part of the Plasmodium of Badhamia utricularis.

The appearance of an active plasmodium of Badhamia utricularis, which, as we have seen, lives and feeds on certain fungi, is shown in fig. 5. It consists of a film of protoplasm, of a bright yellow colour, varying in size up to a foot or more in diameter. It is traversed by a network of branching and anastomosing channels, which divide up and are gradually lost as they approach the margin where the protoplasm forms a uniform and lobate border. Elsewhere the main trunks of the network may lie free with little or no connecting film between them and their neighbours. The plasmodia of other species, which live in the interstices of decaying vegetable matter, are less easily observed, but on emerging on the surface prior to spore formation they present an essentially similar appearance. There is, however, great variety in the degree of concentration or expansion presented by plasmodia, in relation with food supply, moisture and other circumstances. The plasmodia move slowly about over or in the substratum, concentrating in regions where food supply is abundant, and leaving those where it is exhausted.

On examining under the microscope a film which has spread over a cover-slip, the channels are seen to be streams of rapidly moving granular protoplasm. This movement is rhythmic in character, being directed alternately towards the margin of an advancing region of the plasmodium, and away from it. As a channel is watched the stream of granules is seen to become slower, and after a momentary pause to begin in the opposite direction. In an active plasmodium the duration of the flow in either direction varies from a minute and a half to two minutes, though it is always longer when in the direction of the general advance over the substratum. When the flow of the protoplasm is in this latter direction the border becomes turgid, and lobes of hyaline protoplasm are seen (under a high magnification) to start forward, and soon to become filled with granular contents. When the flow is reversed, the margin becomes thin from the drainage away of its contents. A delicate hyaline layer invests the plasmodium, and is apparently less fluid than the material flowing in the channels. The phenomena of the rhythmic movement of the protoplasm are not inconsistent with the view that they result from alternating contraction and relaxation of the outer layer in different regions of the plasmodium, but any dogmatic statement as to their causation appears at present inadvisable.

Fig. 6.

𝑎, Part of a stained Plasmodium of Badhamia utricularis.

𝑛, Nuclei.

𝑏, Nuclei, some in process of simple (amitotic) division.

𝑐, Part of a Plasmodium in which the nuclei are in simultaneous mitotic division.

𝑑-𝑓, Other stages in this process.

Minute contractile vacuoles may be seen in great numbers in the thin parts of the plasmodium between the channels. In stained preparations nuclei, varying (in Badhamia utricularis) from 2·5 to 5 micromillimeters in diameter, are found abundantly in the granular protoplasm (fig. 6, 𝑏). They contain a nuclear reticulum and one or more well-marked nucleoli. In any stained plasmodium some nuclei may be found, as shown in the figure 𝑏, which appear to be in some stage of simple (amitotic) division, and this is, presumably, the chief mode in which the number of the nuclei keeps pace with the rapidly growing plasmodium. There is, however, another mode of nuclear division in the plasmodium which has hitherto been observed in one recorded instance (19, p. 541), the mitotic (fig. 6, 𝑐-𝑓), and this appears to befall all the nuclei of a plasmodium simultaneously. What the relation of these two modes of nuclear division may be to the life-history is obscure.

That the amitotic is the usual mode of nuclear division is indicated by the very frequent occurrence of these apparently dividing nuclei and also by the following experiment. A plasmodium of Badhamia utricularis spreading over pieces of the fungus Auricularia was observed to increase in size about fourfold in fourteen hours, and during this time a small sample was removed and stained every quarter of an hour. The later stainings showed no diminution in the number of nuclei in proportion to the protoplasm, and yet none of the sample showed any sign of mitotic division (20, p. 9). It would appear therefore that the mode of increase of the nuclei during this period was amitotic.

Prowazek (28) has recently referred to nuclear stages, similar to those here regarded as of amitotic division, but has interpreted them as nuclear fusions. He does not, however, discuss the mode of multiplication of nuclei in the plasmodium.

In the group of the Calcareae, granules of carbonate of lime are abundant in the plasmodia, and in all Mycetozoa other granules of undetermined nature are present. The colour of plasmodia varies in different species, and may be yellow, white, pink, purple or green. The colouring matter is in the form of minute drops, and in the Calcareae these invest the lime granules.

Nutrition.—The plasmodium of Badhamia utricularis, advancing over the pilei of suitable fungi, feeds on the superficial layer dissolving the walls of the hyphae (17). The protoplasm may be seen to contain abundant foreign bodies such as spores of fungi or sclerotium cysts (vide infra) which have been taken in and are undergoing digestion. It has been found experimentally (11) that pieces of coagulated proteids are likewise taken in and digested in vacuoles. On the other hand it has been found that plasmodia will live, ultimately producing sporangia, in nutrient solutions (9).^[5] It would appear therefore that the nutrition of plasmodia is effected in part by the ingestion of solid foodstuffs, and in part by the absorption of material in solution, and that there is great variety in the complexity of the substances which serve as their food.

Fig. 7.—Section of the Plasmodium of Badhamia utricularis when passing into the condition of sclerotium.

𝑛, The nuclei contained in the young sclerotial cysts.

Sclerotium.—As the result of drought, the plasmodium, having become much denser by loss of water, passes into the sclerotial condition. Drawing together into a thickish layer, the protoplasm divides up into a number of distinct masses, each containing some 10 to 20 nuclei, and a cyst wall is excreted round each mass (fig. 7). The whole has now a hard brittle consistency. In this state the protoplasm will remain alive for two or three years. On the addition of water the cyst walls are ruptured and in part absorbed, their contents join together, and the active streaming condition of the plasmodium is resumed. It is to be noted, however, that the sclerotial condition may be assumed under other conditions than dryness, and sclerotia may even be formed in water.

The existence of the sclerotial stage affords a ready means of obtaining the plasmodium for experimental purposes. If a cultivation of the plasmodium of Badhamia utricularis on suitable fungi (Stereum, Auricularia) is allowed to become partially dry the plasmodium draws together and would, if drying were continued, pass into the sclerotial stage on the fungus. If now strips of wet blotting-paper are placed so as to touch the plasmodium, the latter, attracted by the moisture, crawls on the blotting-paper. If this is now removed and allowed to dry rapidly, the plasmodium passes into sclerotium on it.^[6] By this means the plasmodium is removed from the partially disintegrated and decayed fungus on which it has been feeding, and a clean sclerotium is obtained, which, as above stated, remains alive for years (21, p. 7). An easy method for obtaining small plasmodia for microscopic examination is to scatter small fragments, scraped from a piece of the hard sclerotium, over cover-slips wetted with rain-water and kept in a moist atmosphere. In twelve to twenty-four hours small plasmodia will be seen spreading on the cover-slips and these may be mounted for observation.

The plasmodial stage ends by the formation of the sporangia. The plasmodium withdraws from the interstices of the material among which it has fed, and emerges on the surface in a diffuse or concentrated mass. In the case of Badhamia utricularis it may withdraw from the fungus on which it has been feeding, or change into sporangia on it. The mode of formation of the sporangia will be described in the case of Badhamia, some of the chief differences in the process and in the structure of the sporangia in other forms being subsequently noticed.

When the change to sporangia begins the protoplasm of the plasmodium becomes gradually massed in discrete rounded lobes, about a half to one millimeter in diameter and scattered in clusters over the area occupied by the plasmodium. The reticulum of channels of the plasmodium becomes meanwhile less and less marked. When the whole of the protoplasm is drawn in to the lobes, the circulation ceases. The lobes are the young sporangia. Meanwhile foreign bodies, taken in with the food, are ejected, and the protoplasm secretes on its outer surface a pellicle of mucoid, transparent substance which dries as the sporangia ripen. This invests the young sporangia, and as they rise above the substratum falls together at their bases forming the stalks; extended over the substratum it forms the hypothallus, and in contact with the rounded surface of the sporangium it forms the sporangium-wall. While the sporangium-wall is formed externally a secretion of similar material occurs along branching and anastomosing tracts through the protoplasm of the sporangium, giving rise to the capillitium. The greater part of the lime granules pass out of the protoplasm and are deposited in the capillitium, which in the ripe sporangia of Badhamia is white and brittle with the contained lime (cf. fig. 8). In this genus some granules are found also in the sporangium-wall. Strasburger concludes that the sporangium-wall of Trichia is a modification of cellulose (29).

Fig. 8.—Sporangia of Badhamia panicea, some intact, others (to left) ruptured, exposing the black masses of spores and the capillitium. The latter is white with deposited lime granules. An empty sporangium is seen above.

It has been stated (16), but the observation requires confirmation, that a fusion of the nuclei in pairs occurs early in the development of the sporangium.

Fig. 9.—Part of a section through a young Sporangium of Trichia varia, showing the mitotic division of the nuclei (n) prior to spore formation. 𝑐, Capillitium thread.		Fig. 10.—Part of a section through a Sporangium of Trichia varia after the spores are formed.
Fig. 11.—Badhamia utricularis. 𝑎, Sporangia. 𝑏, Capillitium and cluster of spores.		Fig. 12.—Physarum nutans. 𝑎, Sporangia. 𝑏, Capillitium threads, with fragment of the sporangium-wall attached, lime knots at the junctions and spores.

At a later stage, after the capillitium is formed, the nuclei undergo a mitotic division which affects all the nuclei of a sporangium simultaneously. This was first described by Strasburger (29). While it is in progress the protoplasm of the sporangium divides, into successively smaller masses, until each daughter nucleus is the centre of a single mass of protoplasm.^[7] These nucleated masses are the young spores. A spore-wall is soon secreted and the sporangium has now resolved itself into a mass of spores, traversed by the strands of the capillitium and enclosed in a sporangium-wall, connected with the substratum by a stalk. As ripening proceeds, the wall becomes membranous and readily ruptures, and the dry spores may be carried abroad on the currents of air or washed out by rain.

Fig. 13.—Chondrioderma testaceum. 𝑎, Group of three Sporangia. 𝑏, Capillitium, fragment of sporangium-wall and spores.		Fig. 14.—Craterium pedunculatum. 𝑎, Two Sporangia, in one the lid has fallen away. 𝑏, Capillitium with lime knots and spores.
Fig. 15.—Didymium effusum. 𝑎, Two Sporangia, one showing the columella and capillitium. 𝑏, Capillitium, fragment of sporangium-wall with carbonate of lime in crystals, and spores.		Fig. 16.—Lepidoderma tigrinum. 𝑎, Sporangium; the crystal-line disks of lime are seen attached to the sporangium-wall. 𝑏, Capillitium and spores.

We may now review some of the main differences in structure presented by the sporangia. They may be stalked or sessile (fig. 13). If the former, the stalk is usually, as in Badhamia utricularis, the continuation of the sporangium-walls (figs. 11 and 12), but in Stemonitis and its allies (figs. 17 and 18) it is an axial structure. A central columella may project into the interior of the sporangium, either in stalked (fig. 15) or sessile (fig. 13) forms.

Fig. 17.—Lamproderma irlaeum. 𝑎, Sporangia. 𝑏, A Sporangium deprived of spores, showing the capillitium and remains of the sporangium-wall.		Fig. 18.—Stemonitis splendens. 𝑎, Group of Sporangia (nat. size). 𝑏, Portion of columella and capillitium, the latter branching to form a superficial network.
Fig. 19.—Dictydium umblicatum. 𝑎, Group of Sporangia, nat. size. 𝑏, A Sporangium after dispersion of the spores.		Fig. 20.—Arcyria punicea. 𝑎, Group of Sporangia. 𝑏, Capillitium. 𝑐, Spore.

The sporangium-wall may be most delicate and evanescent (fig. 17), or consist of a superficial network of threads (fig. 18), which in Dictydium (fig. 19) present a beautifully regular arrangement.

In Chondrioderma (fig. 13) the wall is double, the inner layer being membranous, the outer thickly encrusted with lime granules. In Craterium the upper part of the sporangium-wall is lid-like and falls away, leaving the spores in an open cup (fig. 14).

The condition of the capillitium is very various. In the Calcarineae the lime may be generally distributed through it (fig. 11), or aggregated at the nodes of the network in “lime-knots” (figs. 12 and 14) or it may be absent from the capillitium altogether. The capillitium attains its highest development in the Calonemineae in which the threads, distinct (in which case they are known as elaters, figs. 9 and 10) or united into a network (fig. 20), present regular thickenings in the form of spiral bands or transverse bars. These threads, altering their shape with varying states of moisture, are efficient agents in distributing the spores. In another group, the Anemineae, the capillitium is absent altogether.

The Didymiaceae are characterized by the fact that the lime, though present in a granular form in the plasmodium, is deposited on the sporangium-wall in the form of crystals, either in radiating groups (fig. 15) or in disks (fig. 16).

Fig. 21.—Fuligo septica. 𝑎, Aethalium. 𝑏, Capillitium threads (with lime-knots) and two spores.		Fig. 22.—Licea flexuosa. 𝑎, Group of Plasmodiocarps. 𝑏, A continuous Plasmodiocarp. 𝑐, Spores.

In most Endosporeae the sporangia are separate symmetrical bodies, but in many genera a form of fructification occurs in which the spores are produced in masses of more or less irregular outline, retaining in extreme cases much of the diffuse character of the plasmodium. With the spores they contain capillitium, but there are no traces of sporangial walls to be found in their interior. They are known as plasmodiocarps (fig. 22). They are characteristic of certain species, but in others they may be formed side by side with separate sporangia from the same plasmodium. There is indeed no sharp line to be drawn between sporangia and plasmodiocarps. On the other hand, the crowded condition of the sporangia of some species forms a transition to the large compound fructifications known as aethalia (fig. 21). These, either in their young stages or up to maturity, retain some evidence of their formation by a coalescence of sporangia, and in addition to the capillitium they are generally penetrated by the remains of the walls of the sporangia which have thus united.

Exosporeae.

From Lankester’s Treatise on Zoology; figs. 𝑎 and 𝑐-ℎ after A. Lister; fig. 𝑏 after Famintzin and Woronin.

Fig. 23.—Ceratiomyxa mucida.

𝑎, Ripe sporophore.

𝑏, Maturing sporophore showing the development of the spores.

𝑐, Ripe spore. Instead of the single nucleus here indicated there should be four nuclei, as in 𝑑.

𝑑, Hatching spore.

𝑒-ℎ, Stages in the development of the zoospores.

It will be convenient to begin our survey of the life-history of Ceratiomyxa, the single representative of the Exosporeae, at the stage at which the plasmodium emerges from the rotten wood in which it has fed. At this stage it has been observed to spread as a film over a slide, and to exhibit the network of channels and rhythmic flow of the protoplasm in a manner precisely similar to that seen in the Endosporeae (20, p. 10). It soon, however, draws together into compact masses, from the surface of which finger-like or antler-like lobes grow upwards. Here too the secretion of a transparent mucoid substance occurs, which is at first penetrated by the anastomosing strands of the protoplasm, but gradually the latter tends more and more to form a reticular and ultimately a nearly continuous superficial investment, covering the mucoid material. The latter eventually dries and forms the exceedingly delicate support of the spores or sporophore (fig. 23, 𝑎).

The investing protoplasm, with its nuclei, having become arranged in an even layer, undergoes cleavage and thus forms a pavement-like layer of protoplasmic masses, each occupied by a single nucleus (fig. 23, 𝑏). Each of these masses now grows out perpendicularly to the surface of the sporophore. As it does so an envelope is secreted, which, closing in about the base forms a slender stalk. The minute mass, borne on the stalk, becomes the ellipsoid spore, surrounded by the spore-wall. In this manner the whole of the protoplasmic substance of the plasmodium is converted into spores, borne on supporting structures (stalks and sporophores), which are formed by secretion of the protoplasm.

In the course of the development of which the external features have now been traced nuclear changes occur of which accounts have been given by Jahn (14) and by Olive (24 and 25). Jahn has shown that prior to the cleavage of the protoplasm a mitotic division of the nuclei takes place, the daughter nuclei of which are those occupying the protoplasmic masses seen in fig. 23 𝑏.^[8] After the spore has risen on its stalk two further mitotic divisions occur in rapid succession, and the four-nucleated condition characteristic of the spore of Ceratiomyxa, is thus attained. The spores, on being brought into water, soon hatch (fig. 23, 𝑑), and the four nuclei contained in them undergo a mitotic division. Meanwhile the protoplasm divides, at first into four, then into eight masses, and the latter acquire flagella, although for some time remaining connected with their fellows (fig. 23, 𝑒-ℎ). On separating each is a free zoospore.

From observation of cultivations of zoospores the impression is that here, as in the Endosporeae, they multiply by binary division, though no exact observations of the process have been recorded. The zoospores lose their flagella and become amoebulae, but the fusion of the latter to form plasmodia has not been directly observed in Ceratiomyxa, although from analogy with the Endosporeae it can hardly be doubted that such fusions occur.

Sorophora.

From Lankester’s Treatise on Zoology; 𝑎 and 𝑏 after Fayod; 𝑐 and 𝑑 after Brefeld from Zopf.

Fig. 24.—𝑎 and 𝑏, Capromyxa protea, slightly magnified.

𝑐 and 𝑑, Polysphondylium violaceum.

𝑐, A young sorus, seen in optical section. A mass of elongated amoebulae are grouped round the stalk, and others are extended about the base.

𝑑, A sorus approaching maturity.

The Sorophora of Zopf (Acrasiae of Van Tieghem) are a group of microscopic organisms inhabiting the dung of herbivorous animals and other decaying vegetable matter. As Pinoy (26) has shown, the presence of a particular species of bacteria with the spores is necessary for their hatching and as the essential food of the amoebulae which emerge from them. There is no flagellate stage, and it is in the form of amoebulae, multiplying by fission, that the vegetative stage of the life-history is passed. At the end of this stage numbers of amoebulae draw together to form a “pseudo-plasmodium.” This appears to be merely an aggregation of amoebulae prior to spore formation. The outlines of the individual amoebulae are maintained, and there is no fusion between them, as in the formation of the plasmodium of the Euplasmodida.

In some genera certain of the amoebulae constituting the pseudo-plasmodium are modified into a stalk (simple in Guttulina and Dictyostelium, branched in Polysphondylium, fig. 24, 𝑑), along which the other units creep to encyst, and become spores at the end or ends of the stalk. In other cases (Copromyxa, fig. 24, 𝑎 and 𝑏) the pseudo-plasmodium is transformed into a mass of encysted spores without the differentiation of supporting structures.

It is not impossible that the Myxobacteriaceae of Thaxter may, as that author suggests, be allied to the Sorophora (30).

Review of the Life-Histories of the Mycetozoa.—The data for a comparison of the life-history of the Mycetozoa with those of other Protozoa in respect of nuclear changes are at present incomplete. At some stage or other we are led by analogy to expect that a division of nuclei would occur in which the number of chromosomes would be reduced by one half, that this would be followed by the formation of gametes, and that the nuclei of the latter would subsequently fuse in karyogamy.

It is clear that both in the Endosporeae and Exosporeae a mitotic division of nuclei immediately precedes spore-formation. This is regarded by Jahn as a reduction division. If this is the case, the zoospores or the amoebulae must in some way represent the gametes. The fusion of the latter to form plasmodia appears to offer a process comparable with the conjugation of gametes, but though the fusion of the protoplasm of the amoebulae has been often observed no fusion of their nuclei (karyogamy) has been found to accompany it. A fusion of nuclei has indeed been described as occurring in the plasmodium, or at stages in the development of the sporangia or sporophores, but in no case can the evidence be regarded as satisfactory.^[9] Until we have clear evidence on this point the nuclear history of the mycetozoa must remain incomplete.

Jahn’s observation of the mitotic division of nuclei preceding spore-formation in Ceratiomyxa gives a fixed point for comparison of the Exosporeae with the Endosporeae. Starting from this division it seems clear that the spore of Ceratiomyxa is comparable with the spore of the Endosporeae except that the nucleus of the former has undergone two mitotic divisions.

Literature.—(1) A. de Bary, “Die Mycetozoen,” Zeitschr. f. wiss. Zool., x. 88 (1860). (2) “Die Mycetozoen,” (2nd ed., Leipzig, 1864). (3) Comparative Morphology and Biology of the Fungi, Mycetozoa and Bacteria, translation (Oxford, Clarendon Press, 1887). (4) O. Bütschli, “Protozoa, Abth. g. Sarcodina,” Bronn’s Thierreich, Bd. i. (5) L. Cienkowski, “Die Pseudogonidien,” Pringsheim’s Jahrbücher, i. 371. (6) “Zur Entwickelungsgeschichte der Myxomyceten,” Pringsheim’s Jahrbücher, iii. 325 (pub. 1862). (7) “Das Plasmodium,” ibid. p. 400 (1863). (8) “Beiträge zur Kenntniss der Monaden,” Arch. f mikr. Anat. i. 203 (1865). (9) J. C. Constantineanu, “Ueber die Entwicklungsbedingungen der Myxomyceten,” Annales mycologici, Vierter Jahrg. (Dec. 1906). (10) A. Famintzin and M. Woronin, “Ueber zwei neue Formen von Schleimpilzen Ceratium hydnoides, A. und Sch., and C. porioides, A. und Sch.,” Mém. de l'acad. imp. d. sciences de St Petersburg, series 7, T. 20, No. 3 (1873). (11) M. Greenwood and E. R. Saunders, “On the Rôle of Acid in Protozoan Digestion,” Jour. of Physiology, xvi. 441 (1894). (12) R. A. Harper, “Cell and Nuclear Division in Fuligo varians,” Botanical Gazette, vol. 30, No. 4, p. 217 (1900). (13) E. Jahn, “Myxomycetenstudien 3. Kernteilung u. Geisselbildung bei den Schwärmern von Stemonitis flaccida, Lister,” Bericht d. deutschen botanischen Gesellschaft, Bd. 22 p. 84 (1904). (14) “Myxomiycetenstudien 6. Kernverschmelzungen und Reduktionsteilungen,” ibid. Bd. 25, p. 23 (1907). (15) W. Saville Kent, “The Myxomycetes or Mycetozoa; Animals or Plants?” Popular Science Review, n.s., v. 97 (1881). (16) H. Kränzlin, “Zur Entwicklungsgeschichte der Sporangien bei den Trichien und Arcyrien,” Arch. f. Protistenkunde, Bd. ix. Heft. 1, p. 170 (1907). (17) A. Lister, “Notes on the Plasmodium of Badhamia utricularis and Brefeldia maxima,” Ann. of Botany, vol. ii. No. 5 (1888). (18) “On the Ingestion of Food Material by the Swarm-Cells of the Mycetozoa,” Journ. Linn. Soc. (Bot.) xxv. 435 (1889). (19) “On the Division of Nuclei in the Mycetozoa,” Journ. Linn. Soc. (Bot.) vol. xxix. (189). (20) “A Monograph of the Mycetozoa,” British Museum Catalogue (London, 1894). (21) “Presidential Address to the British Mycological Society,” Trans. Brit. Mycological Soc. (1906). (22) A. and G. Lister, “Synopsis of the Orders, Genera and Species of Mycetozoa,” Journal of Botany, vol. xlv. (May 1907). (23) E. W. Olive, “Monograph of the Acrasiae,” Proc. Boston Soc. of Nat. History, vol. xxx. No. 6 (1902). (24) “Evidences of Sexual Reproduction in the Slime Moulds,” Science, n.s., xxv. 266 (Feb. 1907). (25) “Cytological Studies in Ceratiomyxa,” Trans. Wisconsin Acad. of Sciences, Arts and Letters, vol. xv., pt. ii. p. 753 (Dec. 1907). (26) E. Pinoy, “Rôle des bactéries dans le développement de certains Myxomycètes,” Ann. de l'institut Pasteur, T. xxi. pp. 622 and 686 (1907). (27) H. Plenge, “Ueber die Verbindungen zwischen Geissel u. Kern bei den Schwärmerzellen d. Mycetozoen,” Verh. d. naturist.-med. Vereins zu Heidelberg, N.F. Bd. vi. Heft 3 (1899). (28) S. von Prowazek, “Kernveränderungen in Myxomycetenplasmodien,” Oesterreich. botan. Zeitschr. Bd. liv. p. 278 (1904). (29) E. Strasburger, “Zur Entwickelungsgeschichte d. Sporangien von Trichia fallax,” Botanische Zeitung (1884). (30) R. Thaxter, “On the Myxobacteriaceae, a new order of Schizomycetes,” Botanical Gazette, xvii. 389 (1892). (31) W. Zopf, “Die Pilzthiere oder Schleimpilze,” Schenk’s Handbuch der Botanik (1887).  (J. J. Lr.) 

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1. Bursulla, a member of Zopf’s Monadina, likewise forms its spores in air.
2. The classification of the Euplasmodida here given is that of A. and G. Lister (22), the outcome of a careful study of the group extending over more than twenty-five years. The writer of this article desires to express his indebtedness to the opportunities he has had of becoming familiar with the work of his father, Mr A. Lister, F.R.S., whose views on the affinities and life-history of the Mycetozoa he has endeavoured herein to summarize.
3. Figures 1, 4, and 11-22 are from the British Museum Guide to the British Mycetozoa. The other figures are from Lankester's Treatise on Zoology, part 1. Introduction and Protozoa. Fascicle 1. Article Mycetozoa.
4. Pinoy states (26) that the spores of Spumaria alba, cultivated with bacteria on solid media, hatch out into amoebae, which under these conditions do not assume the flagellate stage. The amoeba from a spore was observed to give rise by three successive divisions to eight amoebulae.
5. A solution which has thus been found favourable contains the following mineral salts: KH₂PO₄, K₂HPO₄, MgSO₄, KNO₃, CA (NO₃)₂, a free acid, and 5% of dextrine.
6. If the plasmodium is slowly dried it is very apt to pass into sporangia.
7. In some genera such as Arcyria and Trichia (illustrated in figs. 9 and 10) the division of the protoplasm does not occur until the nuclei have undergone this division. The protoplasm then divides up about the daughter nuclei to form the spores.
8. Jahn (14) described two mitotic divisions at this stage, but in “Myxomycetenstudien 7-Ceratiomyxa,” Ber. deut. bot. Gesellsch. xxvi. a (1908) he shows that only one mitotic division occurs in the maturing sporophore prior to c eavage. Olive gives a preliminary account of a fusion o nuclei prior to cleavage, but as he has not seen the mitotic division which certainly occurs at this stage his results cannot be accepted as secure.
9. In the work cited in the last footnote Jahn described a fusion of nuclei as occurring in Ceratiomyxa at the stage at which the plasmodium is emerging to form sporophores. Jahn was at first inclined to regard this union as the sexual karyogamy of the life-cycle, but the writer learns by correspondence (July 1910) that he is inclined to regard this fusion as pathological, and to look for the essential karyogamy elsewhere.