FACTS AND FACTORS OF DEVELOPMENT 53 1 



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 con- 

 stantly increasing number of cells. During the earlier divisions there 



A 



som 



som 

 torn 



V 



P 



Fig. 11, A and B. Two Later Stages in the Development of AmpMoxus, show- 

 ing 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; exit, 

 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 im- 

 portant 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 differen- 

 tiations 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 



