THE FEMALE 



57 



Oogenesis — Maturation of the Egg. 



The maturation process can best be described as it begins, imme- 

 diately after the normal breeding season in the spring. At this time the 

 ovary has been freed of its several thousand mature eggs and con- 

 tains only oogonia with no pigment and little, if any, yolk. Even 

 at this early stage each cluster of oogonia represents a future ovarian 

 unit, consisting of many follicle cells and one ovum. There has been 

 no way to determine which oogonium is to be selected for maturation 

 into an ovum and which will give rise to the numerous follicle cells 

 that act as nurse cells for the growing ovum. It is clear, however, that 

 both follicle cells and the ovum come from original oogonia. All ova 

 develop from oogonia which divide repeatedly. These pre-maturation 

 germ cells divide by mitosis many times and then come to rest, dur- 

 ing which process there is growth of some of them without nuclear 

 division. These become ova while those that fail to grow become 

 follicle cells. However, there are pre-prophase changes of the nucleus 

 of the prospective ovum comparable to the pre-prophase changes in 

 spermatogenesis. The majority of oogonia, therefore, never mature 

 into ova, but become follicle cells. 







11 



' i V' 











9 



Prophases of the heterotypic division in the female (ovary of tadpole). (1) 

 Nucleus of oogonium. (2) Leptotene. (3) Synaptene. (4, 5) Contraction figures. 

 (6) Pachytene. (7) Later pachytene, multiplication of nucleoli. (8, 9) Diplo- 

 tene: the chromatin filaments are becoming achromatic; granules of chromatin 

 are being deposited on the nucleoli. (Courtesy, Jenkinson: "Vertebrate Em- 

 bryology," Oxford, The Clarendon Press.) 



