becomes arranged in the manner ordinarily spoken of as cellular. This division of the nucleus is effected by the process called Cleavage. binary fission; that is to say, it first divides into two, then each of these divides simultaneously again into two, giving four nuclei; each of these after a pause again simultaneously divides into two. So the process continues for some time until the ovum becomes possessed of a large number of nuclei, all of which have proceeded from the original nucleus by a series of binary fissions. This division of the nucleus, which constitutes the essential part of the cleavage of the ovum, continues through the whole of life, but it is only in the earliest period that it is distinguished by a distinct name and used to characterize a stage of development. The nuclear division of cleavage is usually at first a rhythmical process; all the nuclei divide simultaneously, and periods of nuclear activity alternate with periods of rest. Nuclear divisions may be said to be of three kinds, according to the accompanying changes in the surrounding protoplasm: (1) accompanied by no visible change, e.g. the multinucleated Protozoon Actinosphaerium; (2) accompanied by a rearrangement of the protoplasm around each nucleus, but not by its division into two separate masses, e.g. the division which results in the formation of a colony of Protozoa; (3) accompanied by the division of the protoplasm into two parts, so that two distinct cells result, e.g. the divisions by which the free wandering leucocytes are produced, the reproduction of uninuclear Protozoa, &c. In the cleavage of the ovum the first two of these methods of division are found, but probably not the third. At one time it was thought that the nuclear divisions of cleavage were always of the third kind, and the result of cleavage was supposed to be a mass of isolated cells, which became reunited in the subsequent development to give rise to the later connexion between the tissues which were known to exist. But in 1885 it was noticed that in the ovum of Peripatus capensis (A. Sedgwick, Quart. Journ. Mic. Science, xxv., 1885, p. 449) the extra-nuclear protoplasm did not divide in the cleavage of the ovum, but merely became rearranged round the increasing nuclei; the continuity of the protoplasm was not broken, but persisted into the later stages of growth, and gave rise to the tissue-connexions which undoubtedly exist in the adult. This discovery was of some importance, because it rendered intelligible the unity of the embryo so far as its developmental processes are concerned, the maintenance of this unity being somewhat surprising on the previous view. On further inquiry and examination it was found that the ova of many other animals presented a cleavage essentially similar to that of Peripatus. Indeed, it was found that the nuclear divisions of cleavage were of the first two kinds just described. In some eggs, e.g. the Alcyonaria, the first nuclear divisions are effected on the first plan, i.e. they take place without at first producing any visible effect upon the protoplasm of the egg. But in the later stages of cleavage the protoplasm becomes arranged around each nucleus and related to it as to a centre. In the majority of eggs, however, the protoplasm, though not undergoing complete cleavage, becomes rearranged round each nucleus as these are formed. The best and clearest instance of this is afforded by many Arthropodan eggs, in which the nucleus of the just-formed zygote takes up a central position, where it undergoes its first division, subsequent divisions taking place entirely within the egg and not in any way affecting its exterior. The result is to give rise to a nucleated network or foam-work of protoplasm, ramifying through the yolk-particles and containing these in its meshes.
In other Arthropodan eggs the cleavage is on the so-called
centrolecithal type, in which the dividing nuclei pass to the cortex
of the ovum, and the surface of the ovum becomes indented with
grooves corresponding to each nucleus. In this kind of cleavage
all the so-called segments are continuous with the central
undivided yolk-mass. It sometimes happens that in Arthropods
the egg breaks up into masses, which cannot be said to have the
value of cells, as they are frequently without nuclei. In other
eggs, characterized by a considerable amount of yolk, e.g. the
ova of Cephalopoda, and of the Vertebrata with much yolk, the
first nucleus takes up an eccentric position in a small patch of
protoplasm which is comparatively free from yolk-particles.
This patch is the germinal disc, and the nuclear divisions are
confined to it and to the transitional region, where it merges
into the denser yolk which makes up the bulk of the egg. At
the close of segmentation the germinal disc consists of a number
of nuclei, each surrounded by its own mass of protoplasm,
which is, however, not separated from the protoplasm round the
neighbouring nuclei, as was formerly supposed, but is continuous
at the points of contact. In this manner the germinal disc has
beecome converted into the blastoderm, which consists of a small
watch-glass-shaped mass of so-called cells resting on, but continuous
with, the large yolk-mass. It is characteristic of this
kind of ovum that there is always a row of nuclei, called the yolk-nuclei,
placed in the denser yolk immediately adjacent to the
blastoderm. These nuclei are continually undergoing division,
one of the products of division, together with a little of the sparse
yolk protoplasm, passing into the blastoderm to reinforce it
(so-called formative cells). The other product of the dividing
yolk-nuclei remains in the yolk, in readiness for the next division.
In this manner nucleated masses of protoplasm are continually
being added to the periphery of the blastoderm and assisting
in its growth. But it must be borne in mind that all the nucleated
masses of which the blastoderm consists are in continuity with
each other and with the sparse protoplasmic reticulum of the
subjacent yolk.
In the great majority of eggs, then, the nuclear division of cleavage is not accompanied by a complete division of the ovum into separate cells, but only by a rearrangement of the protoplasm, which produces, indeed, the so-called cellular arrangement, and an appearance only of separate cells. But there still remain to be mentioned those small eggs in which the amount of yolk is inconsiderable, and in which division of the nuclei does appear to be accompanied by a complete division of the surrounding protoplasm into separate unconnected cells—ova of many Annelida, Mollusca, Echinoderma, &c., and of Mammalia amongst Vertebrata. In the case of these also (G. F. Andrews, Zool. Bulletin, ii., 1898) it has been shown that the apparently separate spheres are connected by a number of fine anastomosing threads of a hyaline protoplasm, which are not easy to detect and are readily destroyed by the action of reagents. It is therefore probable that the divisions of the nuclei in cleavage are in no case accompanied by complete division of the surrounding protoplasm, and the organism in the cleavage stage is a continuous whole, as it is in all the other stages of its existence.
Of late years a great number of experiments have been made to discover the effects of dividing the embryo during its cleavage, and of destroying certain portions of it. These experiments have been made with the object of testing the view, held by some authorities, that certain segments Division of embryo. are already set apart in cleavage to give rise to certain adult organs, so that if they were destroyed the organs in question could not be developed. The results obtained have not borne out this view. Speaking generally, it may be said that they have been different according to the stage at which the separation was effected and the conditions under which the experiment was carried out. If the experiment be made at a sufficiently early stage, each part, if not too small, will develop into a normal, though small, embryo. In some cases the embryo remained imperfect for a certain time after the experiment, but the loss is eventually made good by regeneration. (For a summary of the work done on this subject see R. S. Bergh, Zool. Centralblatt, vii., 1900, p. 1.)
The end of cleavage is marked by the commencement of the differentiation of the organs. The first differentiation is the formation of the layers. These are three in number, being called respectively the ectoderm, endoderm and mesoderm, or, in embryos in which at their first The layer theory. appearance they lie like sheets one above the other, the epiblast, hypoblast and mesoblast. The layers are sometimes spoken of as the primary organs, and their importance lies in the fact that they are supposed to be generally homologous throughout the series of the Metazoa. This view, which is based partly
