THE ORGANS OF THE MIDDLE GERM-LAYER. 



355 



through disassociation of the cells. In this manner there has now 

 arisen between epidermis and parietal middle layer a longitudinal 

 canal, which stretches over several segments of the tr^nk and com- 

 municates with the body-cavity by means of several successive openings, 

 the pronephric funnels (fig. 194 vri). At one place the pronephric 

 duct comes close up to the epidermis 

 and fuses with it (fig. 198 vg). Al- 

 though an actual opening is' never 

 formed here, still, supported by this 

 fact, one may express the conjecture 

 that originally the pronephros in Ver- 

 tebrates opened out at a point far 

 forward on the body (VAN WLJHE, 

 RUCKERT). 



A short time after its formation the 

 fundament undergoes in its anterior 

 half a complete degeneration ; the pos- 

 terior half, on the contrary, is further 

 developed and enlarges, but remains 

 in connection with the body-cavity by 

 means of a single funnel only (fig. 194 

 vn), either because, as VAN WIJHE as- 

 serts, the several funnels are fused into 

 u single one, or because, in accordance 

 with the account of RUCKERT, all the 

 funnels except a single one become 

 closed and degenerate. 



In the Amphibia, with which the 

 Bony Fishes exactly agree in this point, 

 the pronephros is established in the 

 most anterior part of the trunk as 

 an organ that is from the beginning 

 hollow (fig. 199). Below the primitive 

 segments, which have already been 

 differentiated into muscle-fibres (m), 

 there appears a groove-like evagination (u) of the parietal layer 

 of the peritoneum, which stretches from in front backward over 

 several somites. By detaching itself from its parent-tissue at 

 several places, and remaining in connection with it at others, it is 

 converted into a longitudinal canal, which in Raiia and Bombinator 

 communicates with the body-cavity by means of three pronephric 



Fig. 199. Cross section through a 

 very young Tadpole of Bombinator 

 in the region of the anterior end of 

 the yolk-sac, after GOETTE. 



a, Fold of the outer germ-layer that 

 is continued into the dorsal fin ; 

 is*, spinal cord ; m, lateral muscle ; 

 as*, outer cell-layer of the muscle- 

 plate ; s, mesenchymatic cells ; 6, 

 transition of the parietal into the 

 visceral middle layer; u, pronephros; 

 /, intestinal cavity; e, entoblast, 

 which is continuous with the mass 

 of yolk-cells (d) ; /', ventral ccecal 

 pouch of the intestine, which be- 

 comes the liver. 



