EARLY EMBRYONIC DEVELOPMENT 



131 



elongation and shape (Fig. 83, C), as the dorsal surface becomes 

 ilat and the ventral convex, while the blastopore is at the 

 posterior end of the dorsal surface. The blastopore soon closes, 

 and the mouth and anus are formed independently later. 



The epiblast cells become ciliated all over the surface, 



so that the embryo rotates within the thin covering which still 



surrounds it. And now all the chief systems of the body begin 



to be marked out. The tubular nervous system develops from 



A B 



Pig. 83. — Three stages in the formation of the gastrula of Ampliioxus. In A the nuclei 

 of the endoderm have been omitted ; C has the dorsal surface uppermost, and the 

 posterior end to the right. (From Korschelt and Heider, after Hatschek.) 



a depression of the epiblast (the medullary plate) in the middle 

 line of the flattened dorsal surface (Fig. 84, A, otjj). The edges 

 of the depressed area grow inwards and unite over the deeper 

 layer of epiblast, which becomes the wall of the neural canal or 

 embryonic nervous system (Fig. 84, D, n) ; and further back 

 these edges of the medullary plate join one another behind the 

 blastopore, so that the latter comes to open into the floor of the 

 neural canal, thus forming the neurenteric canal (Fig. 85, A, 

 en). Anteriorly the neural canal (n) opens to the exterior for 

 some time by the neuropore. 



The hypoblastic walls of the archenteron give off a long 

 median dorsal groove which becomes the notoohord (Fig. 84, 

 C and D, ch) ; and also an anterior pouch and certain lateral pairs 



