iv GENITAL OEGANS 273 



degenerate and in many cases they appear to be shed into the 

 splanchnocoele leaving behind them the spaces or follicles in which 

 they lay walled in by indifferent cells. The meaning of this pheno- 

 menon is obscure but a suggestion is made regarding it below. 



A period of active mitotic division of the gonocytes now sets in 

 which leads to the formation of solid masses of gonocytes projecting 

 down into the interior of the young genital gland (Fig. 141, E). 

 These gonocytes are the ancestral oogonia or spermatogonia as 

 the case may be. 



Up till now the genital fold has been spoken of as if it were 

 merely a fold of the coelomic epithelium. As a matter of fact the 

 fold very soon becomes invaded along its attached edge by immigrant 

 mesenchyme cells forming a solid connective tissue core or frame- 

 work which serves both a supporting and later, when it develops 

 blood-vessels, a nutritive function to the developing germ-cells. The 

 penetration of nests of gonocytes into this in the form of solid down- 

 growths of the germinal epithelium may be interpreted as represent- 

 ing an ancestral increase in area of the fertile portions of the germinal 

 epithelium — the increase being originally brought about by the 

 formation of hollow downgrowths into the vascular stroma, and these 

 downgrowths having secondarily lost their cavities. The otherwise 

 mysterious degeneration and shedding of large numbers of gonocytes 

 already referred to may probably be regarded as a means of providing 

 room for the localized parts of germinal epithelium which undergo 

 this active proliferation. 



Ueinogbnital Network. — A characteristic feature of the Verte- 

 brate is the set of tubular channels — vasa efferentia — which in 

 most of its subdivisions connects the testis with the opisthonephros 

 and is frequently to be recognized in more or less vestigial form in 

 the female sex as well. 



Apart from variations in detail, these channels may be said to 

 pass from the cavity of the testis to the cavities of the Malpighian 

 bodies of the opisthonephros. They are clearly recognizable during 

 early stages of development as solid strands of cells lying within the 

 genital fold (Fig. 141, D, gs), the cavity in their interior develop- 

 ing secondarily. As regards their first origin the majority of 

 observers state that they are first recognizable at their renal ends 

 and have therefore interpreted them as outgrowths from the coelomic 

 epithelium of the Malpighian body, i.e. from the nephrotome. 

 Other observers seeing them make their appearance gradually in 

 the core of the genital fold and reaching the Malpighian body 

 secondarily regard them as differentiating in situ from the mesen- 

 chyme, while still others have adduced evidence in favour of the cells 

 which give rise to the strands being budded off from the peritoneal 

 epithelium close to the attached base of the genital fold. The 

 disparity between the statements of different observers is most 

 reasonably to be attributed to actual variation in the mode of 

 development. It may be assumed that originally the connexions 

 VOL. II T 



