4 MEDULLARY CAXAL. 



neural and alimentary tracts becomes interrupted when the caudal 

 fin appears and the anus is formed. The neural canal then 

 extends round the end of the notochord to the ventral side, but 

 subsequently retreats to the dorsal side and terminates in a slight 

 dilatation. 



In the formation of the medullary canal there are two points 

 deserving notice — viz, (1) the connection with the blastopore ; (2) the 

 relation of the walls of the canal to the adjoining epiblast. With 

 reference to the first of these points it is clear that the fact of 

 the blastopore opening on the floor of the neural canal causes a 

 free communication to exist between the archenteron or gastrula 

 cavity and the neural canal ; and that, so long as the anterior pore 

 of the neural canal remains open, the archenteron communicates 

 indirectly with the exterior (vide fig. 2 E). It must not however 

 be supposed (as has been done by some embryologists) that the pore 

 at the front end of the neural canal represents the blastopore carried 

 forwards. It is even probable that what Kowalevsky describes as 

 the carrying of the blastopore to the dorsal side is really the com- 

 mencement of the formation of the neural canal, the walls of which 

 are continuous with the lips of the blastopore. This interpretation 

 receives support from the fact that at a later stage, when the 

 neural and alimentary canals become separated, the neural canal 

 extends round the posterior end of the notochord to the ventral side. 

 The embryonic communication between the neural and alimentary 

 canals is common to most Chordata ; and the tube connecting them 

 will be called the neurenteric canal. It is always formed in 

 fundamentally the same manner as in Amphioxus. With reference 

 to the second point it is to be noted that Amphioxus is exceptional 

 amongst the Chordata in the fact that, before the closure of the neural 

 groove, the layer of cells which will form the neural tube becomes com- 

 pletely separated from the adjoining epiblast (fig. 3 A), and forms a 

 structure which may be spoken of as the medullary plate ; and that 

 in the closure of the neural canal the lateral epiblast forms a complete 



Fig. 3. Sections of an Amphioxus embryo at three stages. (After Kowalevsky.) 



A. Section at gastrula stage. 



B. Section of an embryo slightly younger than that represented in fig. 2 D, 



C. Section through the anterior part of an embryo at the stage represented in 

 fig. 2 E. 



np. neural plate; nc. neural canal; men. archenteron in A and B, and mesenteron 

 in C: ch. notochord; so. mesoblastic somite. 



