ESSENTIAL FEATURES OF SEXUAL PROCESS 1351 



which, develops into a functional spermatozoon. In the nuclear 

 changes preparatory to the first division, the spireme, when it breaks 

 up, gives rise to only half the normal number of chromosomes. Thus 

 if the somatic number of chromosomes were four we should find in the 

 spermatocyte, after the breaking up of the spireme, only two chromo- 

 somes. These take up their position at the equator of the achromatic 

 spindle and then divide, but the division is effected, not by splitting 

 of the double chromosome, but by transverse division. Each chromo- 

 some breaks into half, one half going to each daughter cell. Since 

 each of the reduced number of chromosomes can be regarded as 

 made up of two normal chromosomes placed end to end or joined 



Primordial germ-cell 



Spermatogonia. 



Primary spermatocyte. 

 Secondary spermatocytes. 



Spermatids. 

 Spermatozoa. 



Division-period (the number of divi- 

 sions is much greater) . 



Growth-period. 



Maturation-period. 



FIG. 555. 



to form a ring, as in Fig. 554, 6, the division in the middle pro- 

 vides for a qualitative difference between the two daughter cells. 

 If we indicate the four normal chromosomes as a, b, c, d, in ordi- 

 nary somatic division each daughter cell will also contain chromo- 

 somes which may be represented as al, 61, cl, dl, and 2, 62, c2, d2. 

 In the spermatocyte the two chromosomes may be represented as 

 ab and cd. When they divide one daughter cell receives a and c, 

 while the other daughter cell receives 6 and d. The second division 

 of these daughter cells takes place generally by splitting of the filaments, 

 so that finally four spermatids are produced (Fig. 555), each containing 

 two chromosomes, two of them containing a and c, while the other two 

 contain 6 and d. In the ovum, during maturation, analogous change? 

 take place. Two successive cell divisions occur as in the formation of 

 spermatozoa, but the daughter cells are of very unequal size. In the 

 first division, the heterotypical division, the chromosomes fuse in 



