THE CELLULAR BASIS 135 



sex cells is known as the division period 

 (Fig. 22). 



(b) The Growth Period. Oocytes and 

 Spermatocytes. — This period of rapid cell di- 

 vision is followed by a period of growth with- 

 out division during which the developing sex 

 cells are called primary oocytes or spermato- 

 cytes. This growth period may be very long 

 in the case of the oocytes, lasting, for example, 

 in the human female from the time of birth to 

 the end of the reproductive period ; during 

 this long time the oocytes in the ovary prob- 

 ably never divide, there are as many of them 

 at birth as at any later time ; during this period 

 of growth the ovarian egg becomes relatively 

 large, in some animals, e. g. birds, the largest 

 of all cells. The growth period of a sper- 

 matocyte lasts for a briefer time than does 

 that of an oocyte so that the former remains 

 relatively small (Fig. 22). 



All of the cell divisions which take place 

 during the division period are of the usual 

 kind, in which every chromosome splits 

 lengthwise into two and the two halves then 

 separate and move to opposite poles of the 



