212 F, M. BALFOUR. 
hypoblast and form the mesoblast. The remainder of the 
lower layer cells form a mass equivalent to the yolk-sac of 
many vertebrates, and are not converted directly into the 
tissues of the animal. 
Another point to be noticed is the different relation of 
epiblast cells to the hypoblast cells at the upper and lower 
side of the mouth of the involution. Above it, on its dorsal 
side, the epiblast and hypoblast are continuous with one 
another. On its ventral side they are primitively not so con- 
tinuous. This is due to the epiblast, as was before mentioned, 
growing round the lower layer cells on the ventral side, 
vide ® 11, and merely remaining continuous with them on 
the dorsal. ‘The importance of these two points will appear 
when we come to speak of other vertebrates. 
The next animal whose development it is necessary to 
speak of is the Frog, and its differences from the mode of 
development are quite easy to follow and interpret. Segmen- 
tation is again not uniform, and results in the formation ofan 
upper layer of smaller cells anda lower one of larger ; in the 
centre is a segmentation cavity. The stage at the close of seg- 
mentation is represented in c1. From the diagram it is apparent 
that lower layer cells occupy a larger bulk than they did in 
the previous animal (Plate X, 8 1), and tend to encroach still 
more upon the segmentation cavity, otherwise the differences 
between the two are unimportant. There are, however, two 
points to be noted. In the first place, although the cells of 
the upper pole are distinguished in the diagrams from the 
lower by their colour, it is not possible at this stage to say 
what will become epiblast and what hypoblast. In the 
second place the cells of the upper pole or epiblast consist 
of two layers—an outer called the epidermic layerand an inner 
called the nervous. In the previous cases the epiblast con- 
sisted of a single layer of cells. The presence of these two 
layers is due to a distinction which, arising in most other 
vertebrates late, in the Frog arises early. In most other ver- 
tebrates in the later stages of development the epiblast con- 
sists of an outer layer of passive and an inner of active cells. 
In the Frog and other Batrachians these two layers become 
distinguished at the commencement of development. 
In the next stage (c11) we find that the involution to form 
the alimentary canal has commenced (a /), but that it is of a 
very different character to involution in the previous case. 
It consists in the growing inwards of a number of cells from 
the point x (c1) towards the segmentation cavity. The cells 
which grow in this way are partly the yellow cells and partly 
the smaller green ones. At first this involuted layer of cells 
