i5o 



Heredity and Environment 



live 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 spermatocyte lasts for a briefer 

 time than does that of an oocyte so that the former remains 

 relatively small (Fig. 41). 



Synapsis. — All of the cell divisions which take place during 

 the division period are of the usual kind, in which every chromo- 

 some splits lengthwise into two and the two halves then separate 

 and move to opposite poles of the spindle where they swell up 



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.) 



