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MANUAL OF ELEMENTARY ZOOLOGY 



form a tube which will become the nerve cord. Its hollow is 

 the neural canal, and the blastopore, which is now known 

 as the neurenteric canal, leads from it to the gut. Eventually 

 the neurenteric canal closes. An opening, known as the 

 neuropore, long remains at the front end of the neural canal 

 and puts it into communication with the olfactory pit 



7 ect 



Fig. 359. — Stages in the gastrulation of Amphioxus. Modified. — 

 After Hatschek. In 7 a half blastula is shown ; 8-11 are sections. 



ach., Archenteron ; Up., blastopore; d,, future dorsal side; eel., ectoderm; end., 

 endoderm ; «./., neural plate. The position- is not the same as in Fig. 358, the 

 large cells being to the right instead of below. 



(p. 338). Meanwhile the hypoblast under the neural plate 

 has also been undergoing a folding, by which it gives rise to 

 the notochord and to the mesoderm, which at this stage is 

 known as mesoblast. The notochord is formed by a median 

 longitudinal groove (Fig. 364, 15) which becomes constricted 

 off from the gut from before backwards, its cells eventually 

 rearranging themselves to form a rod (Fig. 362) and becoming 



