118 



EMBRYOLOGY. 



is produced 011 both sides of the chorda the gastral mesoderm, and 

 that which arises from the blastopore the peristomal mesoderm 

 (RABL). 



ch 



mf 



mp 



ak 



mk l 



Ui 



mk* 



C 



mp 

 ak 



Ih 



mk" 



ik 



ch 



Fig. 79. Three cross sections from a series through an egg on which the medullary ridges begin 

 to appear. The sections illustrate the development of the chorda out of the chordal 

 fundament, and the constricting off of the two halves of the middle germ-layer. 



ak, Outer, ik, inner germ-layer ; mk l , parietal, mk", visceral lamella of the middle germ-layer ; 

 mp, medullary plate ; mf, medullary folds ; cJt, chorda; Ih, body-cavity. 



The further development of the fundaments of mesoderm, chorda, 

 and intestine, which subsequently become entirely separated from 

 one another at the places where they now remain in connection, 

 causes the agreement with the conditions found in Amphioxus to 



