iv OVAEY 275 



the germinal epithelium the most conspicuous feature is of course 

 the immense increase in size, accompanied by the storing up of yolk 

 in the cytoplasm, exhibited by those gonocytes which are to become 

 functional eggs. Synchronously the indifferent cells of the germinal 

 epithelium in their neighbourhood become converted into follicular 

 cells, having for their main function the ministering to the metabolic 

 needs of the growing egg-cells. The intervening portions of germinal 

 epithelium, which do not undergo this modification, retain their 

 germinal character and provide successive batches of eggs in 

 successive breeding seasons. 



The cellular strands of the urinogenital network assume, as in 

 the male, a tubular form, their wall becoming a cubical epithelium. 

 As in the male the ovarian ends of these channels come into close 

 relation with one another and fuse to form a central canal. In the 

 Anura the fusion together to form an axial cavity appears to be 

 less complete than in the male, a number of isolated central 

 cavities being formed one behind the other. Fusions which take 

 place later lead merely to reduction in the number of these central 

 spaces (in Rana from about 12-15 down to about 5-7 — Bourn). With 

 further development, and as the functional egg-cells increase in 

 size, the epithelial walls of these spaces become thin and flattened. 

 Eventually they become pressed together and the cavity is reduced 

 to a mere slit. The portions of the tubes lying nearer to the 

 attachment of the ovary become vestigial. 



The presence of these axial cavities in the ovary, homologous 

 with the central canal into which the microgametes are shed in the 

 male, is of great morphological interest. It suggests the possibility 

 that at one time the eggs were shed into this central space and 

 therefore that the condition now holding in the Vertebrata, where 

 the eggs are shed into the open splanchnocoele, is to be interpreted 

 as a reversion to, rather than a persistence of, the primitive method 

 of shedding the eggs. 



Leaving on one side the elaboration of histological detail 

 which is not dealt with in this volume, the development of ovary 

 and testis shows in its main features great uniformity throughout 

 the gnathostomatous Vertebrates, and may therefore be dismissed 

 with a few general remarks. Everywhere we see the gonad consist- 

 ing at an early stage of a localized patch of coelomic epithelium in 

 contact with nutritive and supporting mesenchyme : everywhere 

 we see this coming to project into the splanchnocoele as a more or 

 less prominent ridge or fold. 



A conspicuous feature is the widespread tendency towards 

 increased localization of the actual functional areas of the germinal 

 epithelium. This is seen on a small scale in the development of cell- 

 nests of gonocytes separated by indifferent or sterile portions : it is 

 seen again in the restriction of fertility to a relatively small antero- 

 posterior part of the genital fold, long progonal and epigonal portions 

 becoming sterile : it is seen again even in the actual differentiated 



