GERM LAYERS. 



77 



mesoderm formation in Triton entirely analogous to its formation in Amphioxus, 

 the solid plate of cells being really two layers enclosing the body cavity, but 

 pressed together by the large amount of yolk. Although the mesoderm de- 

 veloped in the region of the blastopore and that which originates more cranially 

 are continuous in front of the blastopore, it is convenient to designate the 

 former the peristomal, the latter the gastral mesoderm. 



The separation of the mesoderm into a dorsal segmented part and a ventral 

 unsegmented part containing the body cavity; the formation of the notochord 

 between the two lateral plates of mesoderm by a constricting off of cells from the 

 entoderm; the closure of the primitive intestine beneath the notochord; the 

 development of the neural groove and folds with their final closure to form the 

 neural tube; and the extension of ectoderm over their surface to form the surface 

 ectoderm (epidermis), are processes quite similar, to the formation of the same 



Epidermis 

 (ectoderm) 



PJimitive segment 



Parietal ~| 



/-mesoderm 

 Visceral j 



Yolk cells 

 (entoderm) 



Fig. 62. — From transverse section through dorsal part of Triton embryo. Herlwig. 



structures in Amphioxus (Fig. 62). Also, as in Amphioxus, the differentia- 

 tion of these structures is more advanced cranially and gradually extends 

 caudally where for some time there exists a growth area in which they are not as 

 yet differentiated. 



In the frog the formation of the mesoderm is sufficiently different from 

 Amphioxus and Triton to make its correlation somewhat difficult. In the frog 

 apparently all trace of mesodermic evagination is lost. Taking a transverse 

 section through the frog's gastrula at a stage when the blastopore is still circular 

 and widely open (Fig. 39), the mesoderm is seen as a flat plate of cells which 

 blends in the medial line with the protentoderm and ventrally with the yoke 

 entoderm (p. 78, Fig. 63). The mesoderm has here arisen apparently by a 

 splitting off of a layer of cells from the protentoderm, the remaining cells of the 

 protentoderm forming the roof of the primitive gut. Beginning at the sides, the 



