i8o 



REDUCTION OF THE CHROMOSOMES 



foregoing account that the numerical reduction of chromatin-;;/rtxsr.y 

 takes place before the polar bodies are actually formed, through the 

 operation of forces which determine the number of tetrads within 

 the germinal vesicle. The numerical reduction is therefore deter- 

 mined in the grandmother-cell of the egg. The actual divisions by 

 which the polar bodies are formed merely distribute the elements of 

 the tetrads. 



2. Reduction in tJie Male. Spcrmat agenesis 



The researches of Platner ('89), Boveri, and especially of Oscar 

 Hertwig ('90, i) have demonstrated that reduction takes place in the 



Primordial germ-cell. 



Spermatogonia. 



-Division-period (the number of divi- 

 sions is much greater). 



'Growth-period. 



Primary spermatocyte. 



Secondary spermatocytes. 



-Maturation-period. 



Spermatids. 

 Spermatozoa. 

 Fig. 90. Diagram showing the genesis of the spermatozoon. [After BOVERI.] 



male in a manner almost precisely parallel to that occurring in the 

 female. Platner first suggested ('89) that the formation of the polar 

 bodies is directly comparable to the last two divisions of the sperm 

 mother-cells (spermatocytes). In the following year Boveri reached 

 the same result in Ascaris, stating his conclusion that reduction in 

 the male must take place in the " grandmother-cell of the sperma- 

 tozoon, just as in the female it takes place in the grandmother-cell 

 of the egg," and that the egg-formation and sperm-formation really 

 agree down to the smallest detail ('90, p. 64). Later in the same 

 year appeared Oscar Hertwig's splendid work on the spermato- 

 genesis of Ascaris, which established this conclusion in the most 

 striking manner. Like the ova, the spermatozoa are descended from 

 primordial germ-cells which by mitotic division give rise to the 



