214 F. M. BALFOUR, 
important of these. This is a result of the previous con- 
tinuity of the epiblast and hypoblast at the point 2, and is 
a feature almost certainly found in Amphioxus, but which I 
will speak of more fully in my account of the Selachian’s 
development. The opening of the blastopore called the 
anus of Rusconi is now quite narrowed, it does not become 
the anus of the adult. It may be noticed that at the front end 
of the embryo the primitive dorsal epithelium of the alimen- 
tary canal is growing in such a way as to form the epithelium 
both of the dorsal and ventral surfaces of the foregut. 
In spite of various features rendering the development of 
the frog more difficult of comprehension than that of most 
other vertebrates, it is easy to see that the step between it 
and Amphioxus is not a very great one, and will very likely 
be bridged over at some future time, when our knowledge of 
the development of other forms becomes greater. 
From the frog to the Selachian is a considerable step, but 
I have again hypothetically sketched a type intermediate 
between them whose development agrees in some important 
points with that of Pelobates fuscus as described by Bam- 
beke. The points of agreement, though not obvious at first 
sight, I shall point out in the course of my description. 
The first stage (D1), at the close of segmentation, deserves 
careful attention. The segmentation cavity by the increase 
of the food yolk is very much diminished in size, and, what 
is still more important, has as it were sunk down so as to 
be completely within the lower layer cells. The roof of 
the segmentation cavity is thus formed of epiblast and lower 
layer cells, a feature which Bambeke finds in Pelodbates fuscus 
and which is certainly found in the Selachians. In the 
Frog we found that the segmentation cavity began to be 
encroached on by the lower layer cells, and from this it is 
only a small step to find these cells creeping still further up 
and forming the roof of the cavity. In the lower layer cells 
themselves we find an important new feature, viz. that 
during segmentation they become divided in two distinct 
parts—one of these where the segments owing to the presence 
of much food yolk are very large, and the other where the 
segments are much smaller. 
The separation between these two is rather sharp. Even 
this separation was foreshadowed in the frog’s egg, in which 
a number of lower layer cells were much smaller and more 
active at the two sides of the segmentation cavity than 
elsewhere. ‘The segmentation cavity at first lies completely 
within the region of the small spheres. The larger cells 
serve almost entirely as food yolk. The epiblast, as is 
