DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



117 



u 



the chorda! 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 tivo mid- 

 dle yerm-layers have arisen 

 in the anterior territory of 

 the embryonic body by a 

 process of evagination at 

 both sides of the chordal 

 jfundament, just as in Am- 

 pkioxus, except that in one 

 case the evayinated cell-mass 

 contains a cavity, in the 

 other case none. 



mk l 

 dp 



ok 

 dz 



ik 

 dh 



A cross section through 



Fig. 78. Cross section through the blastopore of an 

 egg of Triton with feebly expressed medullary 

 groove. 



ak, Outer, ik, inner germ-layer ; mk l , parietal, mk'*, 

 visceral lamella of the middle germ-layer; u, 

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

 intestinal cavity. 



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 lamellse. Of these the outer 

 (mk l ) 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 



