274 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



of the genital portion of the peritoneal epithelium with the peri- 

 toneal funnels, or with the nephrostomes, were in the form of 

 open ciliated grooves or gutters on the surface of the peritoneum, 

 that later on these became closed in to form tubular channels, 

 and that in actual ontogenetic development in the modern 

 amphibia the development from the coelomic epithelium has 

 become obscured except for traces now at one end now at the other. 



At their distal ends the cell-strands in the male can be traced 

 gradually farther and farther into the genital fold until they come 

 into immediate relationship with the cell-nests of gonocytes. In 

 the female of Urodela and Anura the strands do not spread so 

 far into the genital fold, nor are they, even in early stages, so well 

 developed as in the male. 



The fatty body is developmentally simply a portion of the 

 genital fold which becomes specialized as a store-house of fat. In 

 Anura it is the progonal portion which undergoes this differentiation 

 while in Urodeles and Gymnophiona the rudiment of the fatty body 

 is continued backwards as a ridge along the mesial face of the genital 

 fold throughout its extent. 



The fat is stored in the connective tissue of the organ, the fat 

 cells being usually interpreted as immigrant mesenchyme cells which 

 have invaded the rudiment by its base of attachment. It has also 

 been suggested that these fat cells are peritoneal in their origin 

 (Abramowicz, 1913) — a suggestion of obvious interest in view of the 

 general tendency in the animal kingdom for potential germ-cells to 

 undergo degeneration in order to provide nourishment for the germ- 

 cells which become functional. 



Testis. — The development of the functional testis out of the 

 genital fold is seen in peculiarly simple and diagrammatic form in 

 the Gymnophiona. Here the strands of the urinogenital network, as 

 they sprout into the interior of the testis, anastomose together along 

 its axis so as to form a central canal — around which, embedded in 

 the stroma of the organ, lie the rounded nests of gonocytes. Fusion 

 takes place between each gonocyte-nest and the wall of the central 

 canal and then each nest develops a cavity in its own interior and 

 becomes a hollow ampulla opening into the canal at its inner end. 



Various modifications of this simple scheme are to be found. In 

 Gymnophiona themselves ampulla -formation becomes suppressed 

 except in localized regions between successive vasa efferentia, so that 

 . intervening portions of the testis are sterile and form merely thin 

 tubular connexions between the bead-like fertile portions. Again 

 the ampullae vary in shape : they may be elongated and tubular 

 (Discoglossus) or, as in the majority of cases, flattened against one 

 another by pressure. The " axial " canal again may lie close to the 

 surface : it may become greatly branched, as in most Urodeles, or 

 may form a complicated network as in most Anura. 



Ovary. — In the differentiation of the ovary (Bouin, 1901) the 

 most important points to be noted are the following. As regards 



