72 



TEXT-BOOK OF EMBRYOLOGY. 



Mesoderm Formation in Amphibians. In Amphibians the formation of 

 the mesoderm is, like gastrulation, modified by the presence of many large yolk 

 cells. Taking for an example the water salamander (Triton), which furnishes 



Blastopore 



Ectoderm 

 Parietal mesoderm 



Visceral mesoderm 



Entoderm 



Primitive gut 

 FIG. 60. From transverse section through Triton embryo at region of blastopore. Hertwig. 



perhaps the simplest type of mesoderm formation in Amphibians, only in the 

 region of the blastopore does the mesoderm formation conform at all closely to 

 that of Amphioxus. In this region the middle germ layer is seen to consist of 

 two lateral evaginations which push out between the entoderm and ectoderm, 



Notochord anlage Neural plate 



Parietal mesoderm 

 Visceral mesoderm 



Primitive gut 

 Entoderm 



FIG. 61. From transverse section through Triton embryo in front of blastopore. Hertwig. 



each containing a cavity, the primitive body cavity (Fig. 60). More cranially 

 the mesoderm grows out laterally between the entoderm and ectoderm, not as 

 two hollow evaginations, but as solid plates of cells which only later separate into 

 two layers and enclose the primitive body cavity (Fig. 61). Hertwig considers 



