Heredity and Environment 



(b) The Growth Period. Oocytes and Spermatocytes. This 

 period of rapid cell division is followed by a period of growth 

 without division during which the developing sex cells are called 

 primary oocytes or spermatocytes. 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 reproduc- 

 tive 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 



FIG. 50. DIFFERENT STAGES IN THE DEVELOPMENT OF THE EGG OF THE 

 RABBITT. A, At the beginning of the growth period showing slender 

 chromatic threads in the nucleus. B, Later stage in which these threads 

 ball up and parallel threads conjugate forming the shorter, thicker thread 

 shown in C. D and E, Later stages showing pairs of chromosomes due 

 to conjugation. F, Later stage in which the distinctness of the chromo- 

 somes is temporarily lost. (After Winiwarter.) 



