DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



117 



the chordal fundament as a single layer, become two layers thick 

 farther down, and thus merge into the more voluminous accumu- 

 lation of yolk-cells, which, in all Amphibian embryos, occupy the 

 ventral side and restrict the gastrula-cavity. They correspond, to 

 continue with our comparison, with the entoderm, whereas the 

 small-celled masses, which, starting from the fundament of the 

 chorda, have crowded themselves out between the entoderm and 

 the outer germ-layer, are comparable with the cells which in Am- 

 phioxus and in our diagram form the wall of the body -sacs, or the 

 middle germ-layer. The 

 conclusion is therefore jus- 

 tified and very obvious, 

 that in Triton the two mid- 

 dle germ-layers have arisen 

 in the anterior territory of 

 the embryonic body by a 

 process of evagination at 

 both sides of the chordal 

 fundament, just as in Ain- 

 phioxus, except that in one 

 case the evaginated cell-mass 

 contains a cavity, in the 

 other case none. 



A cross section through 

 the blastopore of the Triton 

 embryo (fig. 78) is to be 

 compared with our second 

 diagram (fig. 75). The 



hollow body-sacs of the latter correspond to the solid cell-bands, 

 which are the fundament of the middle germ-layer. Near the 

 blastopore (u) they are split into two lamellae. Of these the outer 

 (mk 1 ) merges, as in our diagram, into the inner layer of the blasto- 

 poric lip, and becomes continuous at the edge of the blastopore with 

 the outer germ-layer (ak) ; the inner lamella (mk 2 ), on the contrary, 

 is connected with the mass of yolk-cells (dz), which lies like a wall in 

 front of the blastopore and even projects into it as the RUSCONIAN 

 yolk-plug (dp). 



Posteriorly to the blastopore, the middle germ-layer stretches itself 

 out for some distance, but here only as a single connected mass. 



According to the region from which the middle germ-layer is de- 

 veloped, we may divide it into two portions, and call that part which 



Fig. 78. Cross section through the blastopore of an 

 egg of Triton with feebly expressed medullary 

 groove. 



ak, Outer, ik, inner germ-layer ; infc 1 , parietal, ml:', 

 visceral lamella of the middle germ-layer; u, 

 blastopore j dz, yolk-cells; dp, yolk-plug; dh, 

 intestinal cavity. 



