5O T. M. ESSENBERG. 



gression. Definitive germ cells originate from peritoneal deriva- 

 tives later and differentiate into functional follicles of the adult 

 ovary. All young females undergo retrogression, of which three 

 classes may be distinguished. In some, hereafter distinguished as 

 class I, formation of definitive follicles begins before retrogression 

 is well advanced. These comprise about 50 per cent, of all young 

 females (see Table IV., p. 68). In others (class 2) no formation 

 of definitive follicles precedes complete degeneration of primordial 

 follicles, but indifferent germ cells are seen in the epithelium be- 

 fore degeneration is complete. In others, again (class 3), retro- 

 gression of primordial germ cells is complete before any germ cells 

 appear in the epithelium. In the first class there is no doubt that 

 development into functional females occurs ; the later history of 

 the other two classes is considered beyond. The ovarian cavity 

 and the oviduct are formed comparatively early in the development 

 of the ovary. 



i. Early Normal Ovary. 



The total length of the young female varies from 9.3 to 16.9 mm. 

 The average form index is 5.02 and the fin ratio is 1.13. 



The indifferent gonad passes into the ovary very gradually. 

 Paired and considerably apart at the beginning, the two ovaries 

 approach each other until they meet medially and fuse into one 

 gonad which is the normal condition of the adult ovary. The 

 external contour of the gonad is very regular and its component 

 cells, the primordial germ cells and the peritoneal cells, are evenly 

 distributed (Fig. 2). The gonad gradually increases in size owing 

 to the multiplication of its component cells, although no mitotic 

 figures have been discovered. This is true of all the material on 

 hand. 



At the time the two gonads have approached each other the pic- 

 ture has changed considerably. The primordial germ cells increase 

 in size and become completely invested by peritoneal cells to form 

 a follicle of one layer of cells thick. The growth of the germ cells 

 is not simultaneous, some grow faster than others, but all the germ 

 cells present in the gonad are subject to a transformation to young 

 ova at this stage or the early phase of the next stage. Blood vessels 

 which are present in the mesentery enter the young ovary in the 

 form of capillaries. 



