THE EMBRYO AT THE TIME OF HATCH INC. 



59 



end of the embryo (Fig. 19, BP). The lateral plates, or neural 

 folds, of the epiblast extend not merely along the edges of the 

 neural plate, but round the sides and posterior lip of the blasto- 

 pore as well ; and by their fusion in the median plane the 

 blastopore becomes roofed over, so that it no longer opens 

 directly to the exterior, but into the hinder end of the neural 

 canal (cf. Figs. 19 and 25). The blastopore thus forms a short 

 tubular channel of communication between the neural canal and 

 the archenteron, and to this channel the name neurenteric canal 

 is given (Fig. 25, NT). 



It is a curious fact, and one the full meaning of which is 

 not yet determined, that for a time the sole communication 

 between the archenteron, or primitive alimentary canal, and the 



NF 



FIG. 25. 



FIG. 26. 



FIGS. 25 and 26. Ampliioxus embryos at the time of hatching. 

 (After Hatschek.) 



x220. 



Fig. 25. The embryo bisected vertically : the left half is represented, as seen from 

 the inner surface. Fig. 26. The embryo bisected horizontally : the ventral half is 

 represented, as seen from above. CE, enterocoel or mesoblastic somite. E, epiblast. 

 G-, archenteron. H, hypoblast. NF, neural fold. NT, neurenteric canal. PC, polar 

 mesoblast cell. 



exterior should be through the central canal of the nervous system. 

 Kowalevsky, who discovered the neurenteric canal in Amphioxus 

 and in the Ascidians, suggested that these relations may possibly 

 be ancestral, and that animals may have existed, or may still exist, 

 in which the nerve-tube fulfilled a non-nervous function, and 

 possibly acted as part of the alimentary canal. Comparative 

 anatomy has not at present, however, given any support to 

 this suggestion. 



The closure of the neural tube, by meeting and fusion of the 

 neural folds, proceeds from behind forwards, so that a section 



