UROGENITAL ORGANS. 



125 



character ; i.e., are divided into segments, which, in harmony 

 with the names given other regions, have been called gonotomes, 

 but the accuracy of these statements has been disputed recently. 

 In the earlier stages certain of the epithelial cells become 

 larger than their fellows ; and these are called the primordial 

 ova and the primordial seminal cells, accordingly as they are to 

 give rise to eggs or spermatozoa. For a considerable length 

 of time one cannot say, from an examination of these primor- 

 dial cells, whether they are to develop into one or the other of 

 these reproductive cells ; but other structures enable us to 

 decide at an early date, in most vertebrates, whether we have 

 to deal with a male or a female, and so between testes and 

 ovaries. 



The ovaries are those gonads which are to give rise to the 

 eggs or ova. Briefly stated, portions of the epithelium cover- 

 ing the gonad sink into 



the deeper portion of 

 the ovary, carrying with 

 them the primordial ova, 

 while the other cells ar- 

 range themselves as fol- 

 licles around these ova. 

 In this position the ova 

 increase rapidly in size, 

 in part by what must 

 be called a devouring of 

 their neighbors, in part 

 by nourishment fur- 

 nished through the fol- 

 licular cells from the 

 richly vascular mesenchyme adjacent. In the mammals these 

 follicles undergo a peculiar modification, and have secured the 

 special name of Graafian follicles. The follicular epithelium be- 

 comes several cells in thickness and then splits, thus forming an 

 internal cavity, filled with fluid, to one side of which the ovum, 

 surrounded by a few follicular cells, remains attached (Fig. i 33). 

 This region is the discus proligerus. When fully formed the 

 follicle rises to the surface of the ovary, and at the proper time, 



Fig. 133. Portion of ovary of cat. b, blood- 

 vessel ; d^ discus proligerus ; ^, nearly mature 

 ovum ; f, follicle epithelium ; 0, clusters of im- 

 mature ova. 



