1208 



PHYSIOLOGY 



spermatocytes, divide twice, giving rise to four daughter cells, the spermatids, 

 each of 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 spermato- 

 cyte, after the breaking up of the spireme, only two chromosomes. 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 chromo- 

 some, but by transverse division. Each chromosome 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 



Primordial germ-cell 



Spermatogonia. 



Primary spcrmatocyte. 

 Secondary spermatocytes. 



Spermatids. 

 Spermatozoa. 



Division-period (the number of divi 

 sions is much greater) . 



Growth-period. 



M a t urat ion-perind. 



FIG. 557. 



placed end to end or joined to form a ring, as in Fig. 556, b, the division in 

 the middle provides for a qualitative difference between the two daughter 

 cells. If we indicate the four normal chromosomes as a, b, o, d, in ordinary 

 somatic division each daughter cell will also contain chromosomes which 

 may be represented as al, 61, cl, dl, and a2, 62, c2, d2. In the sperma- 

 tocyte 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 b 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. 557), each containing two chromosomes, two of them con- 

 taining a and c, while the other two contain b and d. In the ovum, during 

 maturation, analogous changes 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 chromo- 

 somes fuse in pairs and then divide as in the spermatocytes so that each of 

 the daughter cells contains one half the somatic number of chromosomes. 

 The larger of the two resulting cells, which retains most of the cytoplasm, 



