Iv EARLY EMBRYONIC DEVELOPMENT es 
elongation and shape (Fig. 83, C), as the dorsal surface becomes 
flat 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 
[SIO/ER 
GABESSISAOS LL 
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Fic. 83.—Three stages in the formation of the gastrula of Amphioxus. 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, mp). 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, ~); 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 (7) 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 notochord (Fig. 84, 
Cand D,ch); and also an anterior pouch and certain lateral pairs 
