3. Cleavage.—When the two germ nuclei (egg nucleus and sperm nucleus) have come into contact after the fertilization of the egg they divide by a complicated process known as mitosis, or indirect nuclear division (Fig. 9). The centrosome, which usually accompanies the sperm nucleus in its passage through the egg, divides and forms a spindle-shaped figure with astral radiations at its two poles (Figs. 7, 8). The chromatin, or stainable substance, of the nucleus, takes the form of threads, the chromosomes (Fig. 9), of which there is a constant number for each species of animal and plant. Each chromosome then splits lengthwise, its two halves moving to opposite ends of the spindle, in which position the daughter chromosomes fuse together to form the daughter nuclei. In this way the chromatin of the egg and sperm nuclei is exactly halved.
After the germ nuclei have divided in this manner the entire egg divides by a process of constriction into two cells (Figs, 10, 28). This is the beginning of a long series of cell divisions, each of them essentially like the first, by which the egg is subdivided successively into a constantly increasing number of cells. During the earlier divisions there
Fig. 11. A and B. Two Later Stages in the Development of Amphioxus, showing the elongation of the embryo in the antero-posterior axis (a p), and formation of the somites (som); neural groove (ng) and neural tube (nt); ect, ectoderm; ent, entoderm; mes, mesoderm; ac, alimentary canal. (After Hatschek.)
is little or no increase in the volume of the egg, consequently successive generations of cells continually grow smaller (Figs. 10, 13, 14, A). This process is known as the cleavage of the egg, and by it the egg is not only split up into a considerable number of small cells, but a much more important result is that the different kinds of protoplasm in the egg become isolated in different cleavage cells, so that these substances can no longer freely commingle. The cleavage cells, in short, come to contain different kinds of stubstance, and thus to differ from one another. The differentiations of the cleavage cells appear much earlier in some forms than in others, but in all cases such differentiations appear during cleavage.
4. Embryogeny.—From this stage onward the course of development
