COMPARISON OF THE GERMINAL LAYERS. 279 



process of invagination in Petromyzon. The points in fig. 170 

 where epiblast, mesoblast and hypoblast are continuous, is 

 homologous with the dorsal lip of the blastopore in Amphioxus. 

 In the course of the invagination the segmentation cavity, as in 

 Amphioxus, becomes obliterated. 



While the above invagination has been taking place, the 

 epiblast cells have been simply growing in an epibolic fashion 

 round the yolk; and by the stage represented in fig. 170 C 

 and D the exposed surface of yolk has become greatly di- 

 minished ; and an obvious blastopore is thus established. Along 

 the line of the growth a layer of mesoblast cells (in'\ continuous 

 at the sides with the invaginated mesoblast layer, has become 

 differentiated from the small cells (fig. 170 A) intermediate 

 between the epiblast cells and the yolk. 



Owing to the nature of the above process of invagination the 

 mesenteron is at first only provided with an epithelial wall on 

 its dorsal side, its ventral wall being formed of yolk-cells 

 (fig. 170). At a later period some of the yolk-cells become 

 transformed into the epithelial cells of the ventral wall, while the 

 remainder become enclosed in the alimentary cavity and 

 employed as pabulum. The whole of the yolk-cells, after the 

 separation of the mesoblast, are however morphologically part of 

 the hypoblast. 



The final fate of the blastopore is nearly the same as in 

 Amphioxus. It gradually narrows, and the yolk-cells which at 

 first plug it up disappear (fig. 170 C and D). The neural groove, 

 which becomes formed on the dorsal surface of the embryo, is 

 continued forwards from the point x in fig. 170 C. On the 

 conversion of this groove into a canal the canal freely opens 

 behind into the blastopore ; and a condition is reached in which 

 the blastopore still opens to the exterior and also into the 

 neural canal fig. 170 D. In a later stage (fig. 172) the external 

 opening of the blastopore becomes closed by the medullary folds 

 meeting behind it, but the passage connecting the neural and 

 alimentary canals is left. There is one small difference between 

 the Frog and Amphioxus in the relation of the neural canal to 

 the blastopore. In both types the medullary folds embrace and 

 meet behind it, so that it comes to occupy a position at the hind 

 extremity of the medullary groove. In Amphioxus the closure 



