284 THE GASTRULA OF ELASMOBRANCHII. 



hypoblast cells into permanent hypoblast, which gradually ex- 

 tends inwards towards the segmentation cavity, and exactly re- 

 presents the course of the invagination whereby in Amphibia 

 the dorsal wall of the alimentary cavity is formed. (3) The ob- 

 literation of the segmentation cavity during the period when the 

 pseudo-invagination is occurring. 



In the next stage there appear more important differences 

 between the two types than in the preceding stages, though here 

 again the points of resemblance predominate. 



Figs. 170 D and 174 C represent longitudinal sections through 

 embryos after the closure of the medullary canal. The neuren- 

 teric canal is established ; and in front and behind the epithelium 

 of the ventral wall of the mesenteron has begun to be formed. 



The mesoblast is represented as having grown in between 

 the medullary canal and the superjacent epiblast. 



There are at this stage two points in which the embryo Elas- 

 mobranch differs from the corresponding Amphibian embryo, 

 (i) In the formation of the neurenteric canal, there is no free 

 passage leading into the mesenteron from the exterior as in 

 Amphibia (fig. 170 D). (2) The whole yolk is not enclosed by 

 the epiblast, and therefore part of the blastopore is still open. 



The difference between Amphibia and Elasmobranchii in the 

 first of these points is due to the fact that in Elasmobranchii, as 

 in Amphioxus, the neural canal becomes first closed behind ; and 

 simultaneously with its closure the lateral parts of the lips of the 

 blastopore, which are continuous with the medullary folds, meet 

 together and shut in the hindmost part of the alimentary tract. 



The second point is of some importance for understanding 

 the relations of the formation of the layers in the amniotic and 

 the non-amniotic Vertebrates. Owing to its large size the whole 

 of the yolk in Elasmobranchii is not enclosed by the epiblast at 

 the time when the neurenteric canal is established ; in other words 

 a small posterior and dorsal portion of the blastopore is shut 

 off in the formation of the neurenteric canal. The remaining 

 ventral portion becomes closed at a later period. Its closure 

 takes place in a linear fashion, commencing at the hind end of 

 the embryo, and proceeding apparently backwards ; though, as 

 this part eventually becomes folded in to form the ventral wall 

 of the embryo, the closure of it really travels forwards. The 



