Facts and Factors of Development 21 



individual chromosomes or chromosomal vesicles through the 

 resting stage it is certain that every chromosome preserves its 

 individuality or identity, and even where this cannot be done the 

 fact that the same number of chromosomes, having the same 

 peculiarities of shape and size come out of a nucleus as went into 

 it, is evidence that here also each chromosome has preserved its 

 identity. 



b. Cleavage of the Egg. After the, entrance of the spermato- 

 zoon into the egg the sperm nucleus moves toward the egg nu- 

 cleus until the two meet when they divide by mitosis (Figs. 4 

 I-L, 5 A-F). 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 

 (Fig. 4). The chromatin, or stainable substance of the egg 

 and sperm nuclei, takes the form of threads or chromosomes 

 (Fig. 5). Each chromosome then splits lengthwise, its two halves 

 moving to opposite ends of the spindle, where the daughter chro- 

 mosomes 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 (Fig. 

 5 F). This is the beginning of a long series of cell divisions, 

 each of them essentially like the first, by which the egg is sub- 

 divided successively into a constantly increasing number of cells. 

 During the earlier divisions there is little or no increase in the 

 volume of the egg, consequently successive generations of cells 

 continually grow smaller (Figs. 9-11). 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 sub- 

 stances can no longer freely commingle. The cleavage cells, in 

 short, come to contain different kinds of substance, and thus to 

 differ from one another. The differentiations of the cleavage 



