THIRD PERIOD OF DEVELOPMENT. 69 



separated at the edges of the furrow, and therefore is 

 cut off long before the edges unite with each other. 

 The formation of the neural canal is distinguished 

 from that of the other vertebrate animals, in which 

 its separation only follows after union of the edges 

 of the neural furrow. In the Amphioxus " the dorsal 

 channel, although on the outside fully covered, is on 

 the inside, beneath this covering, still open." The 

 growth of the edges of the neural furrow begins 

 at the posterior end, where the gastrula-mouth is at 

 once enclosed, and advances towards the anterior end, 

 where an opening l is found to persist. 



The remains of the gastrula-mouth continue accord- 

 ingly as an opening between archenteron and neural 

 canal, and this opening is still to be observed even in 

 much later stages. This is the neuro-enteric canal 

 which is generally typical in the development of the 

 vertebrate animals. 



Together with the formation of the neural canal 

 there appears the formation of the rnesoblast in the 

 form of mesoblastic somites. In the posterior part 

 of the hypoblast there arise two lateral longitudinal 

 folds which represent the beginnings of the mesoblast. 



1 Neuropore. 



