in the direction of the cord. This latter structure is an incipient
nerve trunk. It can be traced outwards into the mesoblastic reticulum,
from the strands of which it is indeed developed, and with
which it is continuous not only at its free end, but also along its
whole course. In this way the nerve trunks are developed—by a
gathering up, so to speak, of the fibres of the reticulum into bundles.
These bundles are generally marked by the possession of nuclei,
especially in their cortical parts, which become no doubt the nuclei
of the nerve sheath, and, in the neighbourhood of the ganglia, of
nerve cells. From this account of the early development of the
nerves, it is apparent that they are in their origin continuous with
all the other tissues of the body, with that of the central nervous
system and with that which becomes transformed into muscular
tissue and connective and epithelial tissues. All these tissues are
developed from the general reticulum, which in the young embryo
can be seen to pervade the whole body, not being confined to the
mesoderm, but extending between the nuclei of the ectoderm and
endoderm, and forming the extra-nuclear, so-called cellular, protoplasm
of those layers. Moreover, it must be remarked that in the
stages of the embryo with which we are here concerned the so-called
cellular constitution of the tissues, which is such a marked feature of
the older embryo and adult, has not been arrived at. It is true,
indications of it may be seen in some of the earlier-formed epithelia,
but of nerve cells, muscular cells, and many kinds of gland cells no
distinct signs are yet visible. This remark particularly applies
to nerve cells, which do not make their appearance until a much later
stage—not, indeed, until some time after the principal nerve trunks
and ganglia are indicated as tracts of pale fibrous substance and
aggregations of nuclei respectively.
The embryos of Elasmobranchs—particularly of Scyllium—are the best objects in which to study the development of nerves. In many embryos it is difficult to make out what happens, because the various parts of the body remain so close together that the process is obscured, and the loosening of the mesoblastic nuclei is deferred until after the nerves have begun to be differentiated. The process may also be traced in the embryos of Peripatus, where the main features are essentially similar to those above described (op. cit. p. 131). The development of the motor nerves has been worked out in Lepidosiren by J. Graham Kerr (Trans. Roy. Soc. of Edinburgh, 41, 1904. p. 119).
To sum up, the development of nerves is not, as has been recently urged, an outgrowth of cell processes from certain cells, but is a differentiation of a substance which was already in position, and from which all other organs of the body have been and are developed. It frequently happens that the young nerve tracts can be seen sooner near the central organ than elsewhere, but it is doubtful if any importance can be attached to this fact, since it is not constantly observed. For instance, in the case of the third nerve of Scyllium the differentiation appears to take place earliest near the ciliary ganglion, and to proceed from that point to the base of the mid-brain.
There are two main methods in which new organs are developed. In the one, which indicates the possibility of physiological continuity, the organ arises by the direct modification of a portion of a pre-existing organ; the development of the central nervous system of the Vertebrata Coelom. from a groove in the embryonic ectoderm may be taken as an example of this method. In the other method there is no continuity which can be in any way interpreted as physiological; a centre of growth appears in one of the parts of the embryo, and gives rise to a mass of tissue which gradually shapes itself into the required organ. The development of the central nervous system in Teleosteans and in other similar exceptional cases may be mentioned as an example of the second plan. Such a centre of growth is frequently called a blastema, and consists of a mass of closely packed nuclei which have arisen by the growth-activity of the nuclei in the neighbourhood. The coelom, an organ which is found in the so-called coelomate animals, and which in the adult is usually divided up more or less completely into three parts, namely, body-cavity, renal organs, generative glands, presents in different animals both these methods of development. In certain animals it develops by the direct modification of a part of the primitive enteron, while in others it arises by the gradual shaping of a mass of tissue which consists of a compact mass of nuclei derived by nuclear proliferation from one or more of the pre-existing tissues of the body. Inasmuch as the first rudiment of the coelom nearly always makes its appearance at an early stage, when the ectoderm and endoderm are almost the only tissues present, and as it then bulks relatively very large and frequently contains within itself the potential centres of growth of other organs, e.g. mesenchymal organs (see above), it has come to be regarded by embryologists as being the forerunner of all the so-called mesodermal organs of the body, and has been dignified with the somewhat mysterious rank which attaches to the conception of a germinal layer. Its prominence and importance at an early stage led embryologists, as has already been explained, to overlook the fact that although some of the centres of growth for the formation of other non-coelomic mesodermal organs and tissues may be contained within it, all are not so contained, and that there are centres of mesodermal growth still left in the ectoderm and endoderm after its establishment. If these considerations, and others like them, are correct, it would seem to follow that the conception implied by the word mesoderm has no objective existence, that the tissue of the embryo called mesoderm, though sometimes mainly the rudiment of the coelom, is often much more than this, and contains within itself the rudiment of many, sometimes of all, of the organs appertaining to the mesenchyme. In thus containing within itself the potential centres of growth of other organs and tissues which are commonly ranked as mesodermal, it is not different from the rudiments of the two other organs already formed, namely, the ectoderm and endoderm; for these contain within themselves centres of growth for the production of so-called mesodermal tissues, as witness the nerve-crest of Vertebrata, the growing-point of the pronephric duct, and the formation of blood-vessels from the hypoblast described for some members of the same group.
In Echinodermata, Amphioxus, Enteropneusta, and a few other groups, the coelom develops from a portion or portions of the primitive enteron, which eventually becomes separated from the rest and forms a variable number of closed sacs lying between the gut and the ectoderm. The number of these sacs varies in different animals, but the evidence at present available seems to show that the maximum number is five—an unpaired one in front and two pairs behind—and, further, that if a less number of sacs is actually separated from the enteron, the rule is for these sacs so to divide up that they give rise to five sacs arranged in the manner indicated. The Enteropneusta present us with the clearest case of the separation of five sacs from the primitive enteron (W. Bateson, Quart. Journ. Mic. Sci. xxiv., 1884). In Amphioxus, according to the important researches of E. W. MacBride (Quart. Journ. Mic. Sci. xl. 589), it appears that a similar process occurs, though it is complicated by the fact that the sacs of the posterior pair become divided up at an early stage into many pairs. In Phoronis there are indications of the same phenomenon (A. T. Masterman, Quart. Journ. Mic. Sci. xliii. 375). In the Chaetognatha a single sac only is separated from the enteron, but soon becomes divided up. In the Brachiopoda one pair of sacs is separated from the enteron, but our knowledge of their later history is not sufficient to enable us to say whether they divide up into the typically arranged five sacs. In Echinodermata the number of sacs separated from the enteron varies from one to three; but though the history of these shows considerable differences, there are reasons to believe that the typical final arrangement is one unpaired and two paired sacs. But however many sacs may arise from the primitive enteron, and however these sacs may ultimately divide up and arrange themselves, the important point of development common to all these animals, about which there can be no dispute, is that the coelom is a direct differentiation of a portion of the enteron.
In the majority of the Coelomata the coelomic rudiment does not arise by the simple differentiation of a pre-existing organ, and there is considerable variation in its method of formation. Speaking generally, it may be said to arise by the differentiation of a blastema (see above), which develops at an early stage as a nuclear proliferation from one or more growth-centres in one or both of the primary layers. It appears in this tissue as a sac or as a series of sacs, which become transformed into the body-cavity (except in the Arthropoda), into the renal organs (with the possible exception, again, of some Arthropoda), and into the reproductive glands. In metamerically segmented animals the
