378 THE GASTRULA OF AMPHIBIA. 



belonging to this group (Bombinator). The food-yolk is here 

 concentrated in what I shall call the lower pole of the egg, which 

 becomes the ventral aspect of the future embryo. The part of 

 the .egg containing the stored-up food-yolk is, as has already 

 been explained in the chapter on segmentation (Vol. II. pp. 94 

 and 95), to be regarded as equivalent to part of those eggs 

 which do not contain food-yolk ; a fact which requires to be 

 borne in mind in any attempt to deal comparatively with the 

 formation of the layers in the Vertebrata. It may be laid down 

 as a general law, which holds very accurately for the Vertebrata, 

 that in eggs in which the distribution of food-yolk is not 

 uniform, the size of the cells resulting from segmentation is 

 proportional to the quantity of food-material they contain. 

 In accordance with this law the cells of the Amphibian ovum 

 are of unequal size even at the close of segmentation. They 

 may roughly be divided into two categories, viz. the smaller 

 cells of the upper pole and the larger of the lower (fig. 170 A). 

 The segmentation cavity (sg) lies between the two, but is 

 unsymmetrically placed near the upper pole of the egg, owing to 

 the large bulk of the ventrally placed yolk-segments. In the 

 inequality of the cells at the close of segmentation the Amphibia 

 stand in contrast with Amphioxus. The upper cells are mainly 

 destined to form the epiblast, and the lower the hypoblast and 

 mesoblast. 



The 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 invagi- 

 nation 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 mesoblast. 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 mesen- 

 teron. It is even better shewn in fig. 171 representing the 



