8 THE MAMMALIAN EGG 



significance and has to do with the combination and rearrangement 

 of genes. Genie reassortment assists adaptive variation within the 

 species, while combination of genes from different individuals 

 makes for integration of the race (see Austin, 1959b). 



In the female mammal, germ-cell multiplication is intense in the 

 later phases of embryonic development, and as a result a large 

 number of oogonia accumulate from which eggs can be derived 

 (Brambell, 1956). By the time of birth or shortly afterwards, the 

 oogonia are found already to have differentiated into primary 

 oocytes in which the nuclei are in the initial stage of the prophase 

 of the first meiotic division (the dictyate stage). Further germ-cell 

 multiplication does not appear to take place and the young animal 

 possesses in its ovaries the stock of oocytes that is to last it for the 

 whole of its reproductive life (see Zuckerman, i960). The stock is 

 a very large one, some estimated numbers being: 160,000 in the rat 

 (Slater and Dornfeld, 1945), 700,000 in the dog (Schotterer, 1928) 

 and 750,000 in man (Block, 1953); but only a fraction of these 

 oocytes survives to ovulation, for large numbers degenerate at 

 various stages of oogenesis and at various times during the animal's 

 life. Thus, in the Levant vole (Microtus guntheri) the number of 

 oocytes per ovary, found to be 23,000 at birth, rose to 54,000 on 

 the 4th day of life and then fell gradually to 14,000 on the 27th day 

 and 8,000 on the 75th day (Bodenheimer and Lasch, 1957). De- 

 generation of oocytes can be greatly hastened by treatment of the 

 animal with ionizing radiations; the degree of effect varies with 

 dose, type of radiation, species, age of animal and stage of develop- 

 ment of the oocytes (Brambell, Parkes and Fielding, 1927a, b; 

 Brambell and Parkes, 1927; Brambell, Fielding and Parkes, 1928; 

 Geller, 1930; Genther, 1931; Desaive, 1940, 1941; Oakberg, 1958, 

 i960; Russell and Freeman, 1958; Mandl, 1959; Russell, Stelzner 

 and Russell, 1959; Russell, Russell, Steele and Phipps, 1959). 



Life History 



Oogenesis is completed with the differentiation of the primary 

 oocyte into a mature egg, a process that is characterized by the 

 occurrence of two co-ordinated chains of events — the development 

 of the follicle, and the growth and maturation of the oocyte (Fig. 7). 

 The first evidence of follicle formation is seen when the early 

 primary oocyte becomes surrounded by a single layer of epithelial 

 cells. The number of layers of surrounding cells increases as the 



