CEPIIALOCIIORDA. 



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hand to the formation of the central nervous system, and on the 

 other to the formation of the notochord and mesoblastic somites 1 . 

 The former structure is developed from the epiblast and the two 

 latter from the hypoblast. 



The formation of the central nervous system commences with the 

 flattening of the dorsal surface of the embryo. The flattened area 

 forms a plate (fig. 2 B and fig. 3 A, up), extending backwards to the 

 blastopore, which has in the meantime passed round to the dorsal 

 surface. The sides of the plate become raised as two folds, which 

 are most prominent posteriorly, and meet behind the blastopore, but 

 shade off in front. The two folds next unite dorsally, so as to convert 

 the previous groove into a canaP the neural or medullary canal. 

 They unite first of all over the blastopore, and their line of junction 



FIG. 2. EMBRYOS OF AJIPHIOXUS. (After Kowalevsky. ) 

 The parts in black with white lines are epiblastic ; the shaded parts are hypoblastic. 



A. Gastrula stage in optical section. 



B. Slightly later stage after the neural plate np has become differentiated, seen as 

 a transparent object from the dorsal side. 



C. Lateral view of a slightly older larva in optical section. 



D. Dorsal view of an older larva with the neural canal completely closed except 

 for a small pore (o) in front. 



E. Older larva seen as a transparent object from the side. 



&/. blastopore (which becomes in D the neurenteric canal); nc. neurenteric canal ; 

 np. neural or medullary plate; no. anterior opening of neural canal; ch. notochord; 

 so 1 , so 11 . firs?t and second mesoblastic somites. 



extends from this point forwards (fig. 2 C, D, E). There is in this 

 way formed a tube on the floor of which the blastopore opens behind, 

 and which is itself open in front. Finally the medullary canal is 

 formed for the whole length of the embryo. The anterior opening 

 persists however for some time. The communication between the 



1 The protovertebrae of most ernbryologists will be spoken of as mesoblastic somites. 

 : The details of this process are spoken of below. 



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