CEPHALOCHORDA. 



5 



even probable that what Kowalevsky describes as the carrying 

 of the blastopore to the dorsal side is really the commencement 

 of the formation of the neural canal, the walls of which are con- 

 tinuous 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 completely 

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



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 ; mes. archenteron in A and B, and mesenteron 

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



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 layer above this plate before the plate itself is folded 

 over into a closed canal. This peculiarity will be easily under- 

 stood from an examination of fig. 3 A, B and C. 



The formation of the mesoblastic somites commences, at 

 about the same time as that of the neural canal, as a pair of 

 hollow outgrowths of the walls of the archenteron. These 



