70 



THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION 



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Fig. 41. Development of primary condition of the Graafian follicle in the opossum 

 ovary. (A) Young oocyte with associated epithelial (granulosa) cells which in (B) 

 have encapsulated the oocyte. (C) Encapsulating granulosa cells have increased in 

 number and are assuming a cuboidal shape. (D) Fully developed condition of the 

 primary Graafian follicle. Cf. secondary condition shown in fig. 42. 



Fig. 42. Secondary conditions of the Graafian follicle in the opossum ovary. Cf. that of 



the rat ovary in fig. 40. 



As these changes are initiated, the associated epithehai cells increase in 

 number and eventually encapsulate the oocyte (fig. 41B). This complex of 

 the oocyte with its surrounding layer of follicle cells is known as an egg follicle. 



3. Primary, Secondary, and Tertiary Follicles of de Graaf 



In the mammalian ovary the developing egg with its associated cells is 

 called the Graafian follicle, so named after the Dutch scientist, Reinier de 

 Graaf (fig. 1 ), who first described this structure in mammals in 1672-1673. 

 De Graaf was in error, partly, for he believed that the whole follicular com- 

 plex was the egg. The mammalian egg as such was first described in 1827 



