THE BEGINNING OF DEVELOPMENT 



31 



chromosomes because it involves the reduction of 

 their number to half. 



The daughter-cells of the reduction division are 

 not the actual germ-cells, but each one has to go 



O 



MATURATION PERIOD 



REDUCTION DIVISION / 



o 



REDUCTION 

 DIVISION 







o b 



O'o 



6 O 



PERIOD OF 

 DIFFERENTIATION 



Fig. 4. — Diagram of the formation of the germ-cells, {a) The tg%. 

 {b) The sperm. The egg mother-cell is built up and furnished with 

 yolk during the maturation period, and then undergoes two divisions, 

 giving four cells of which three are very small and die. The sperm 

 mother-cell divides twice and all four resulting cells are transformed 

 into sperms during the period of differentiation. 



through one ordinary division before it is ready, 

 giving a total of four germ-cells from each cell which 

 started the reduction process. Actually, in the forma- 

 tion of the eggs three of these four degenerate and 

 never function as eggs, while the remaining one has to 

 undergo a period of ripening when it is supplied with 

 the yolk which it will require. This ripening usually 

 happens in the middle of the reduction division, 

 which therefore takes a very long time (Fig. 4). 



