232 THE GA8TRULA OF AMPHIBIA. 



upper cells are mainly destined to form the epiblast, and the lower 

 the hypoblast and mesoblast. 



Tlie next change which takes place is an invagination, the earliest 

 traces of which are observable in fig. 170 A. The invagination is not 

 however so simple as in Amphioxus. Owing in fact to the presence 

 of the food-yolk it is a mixture of invagination by epibole and by 

 embole. 



At the point marked x in fig. 170 A, which corresponds with the 

 future hind end of the embryo, and is placed on the equatorial line 

 marking the junction of the large and small cells, there takes place 

 a normal invagination, which gives rise solely to the hypoblast of 

 the dorsal wall of the alimentary tract and to part of the dorsal meso- 

 blast. The invaginated layer grows inwards from the point x along 

 what becomes the dorsal side of the embryo ; and between it and the 

 yolk-cells below is formed a slit-like space (fig. 170 B and C). This 

 space is the mesenteron. It is even better shewn in fig. 171 re- 

 presenting the process of invagination in Petromyzon. The point x 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 Amphi- 

 oxus, 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 and D the exposed surface 

 of yolk has become greatly diminished ; and an obvious blastopore is 

 thus established. Along the line of the growth a layer of mesoblast 

 cells {m), 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 epi- 

 thelial 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 Amphi- 

 (jxus. 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 neiiral 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 



