286 THE GASTRULA OF THE SAUROPSIDA. 



gradually narrows till it becomes completely enveloped in the 

 medullary folds (fig. 175 A). 



On the formation of the neurenteric canal the body of the 

 embryo Elasmobranch becomes gradually folded off from the 

 yolk, which, owing to its great size, forms a large sack appended 

 to the ventral side of the body. The part of the somatopleure, 

 which grows round it, is to be regarded as a modified portion of 

 the ventral wall of the body. The splanchnopleure also enve- 

 lops it, so that, morphologically speaking, the yolk lies within 

 the mesenteron. 



The Teleostei, so far as the first formation of the layers is 

 concerned, resemble in all essential features the Elasmobranchii, 

 but the neurenteric canal is apparently not developed (?), owing 

 to the obliteration of the neural canal ; and the roof of the seg- 

 mentation cavity is formed of epiblast only. 



In the preceding pages I have attempted to shew that the 

 Amphibia, Acipenser, Petromyzon, the Elasmobranchii and the 

 Teleostei agree very closely in the mode of formation of the 

 gastrula. The unsymmetrical gastrula or pseudo-gastrula which 

 is common to them all is, I believe, to be explained by the form 

 of the vertebrate body. In Amphioxus, where the small amount 

 of food-yolk present is distributed uniformly, there is no reason 

 why the invagination and resulting gastrula should not be sym- 

 metrical. In true Vertebrates, where more food-yolk is present, 

 the shape and structure of the body render it necessary for the 

 food-yolk to be stored away on the ventral side of the alimen- 

 tary canal. It is this fact which causes the asymmetry of the 

 gastrula, since it is not possible for the part of the ovum, which 

 will become the ventral wall of the alimentary tract, and which 

 is loaded with food-yolk, to be invaginated in the same fashion 

 as the dorsal wall. 



Sauropsida. The comparison of the different types of the 

 Ichthyopsida is fairly simple, but the comparison of the Sauro- 

 psida with the Ichthyopsida is a far more difficult matter. In all 

 the Sauropsida there is a large food-yolk, and the segmentation 

 agrees closely with that in the Elasmobranchii. It might have 

 been anticipated that the resemblance would continue in the 

 subsequent development. This however is far from being the 



