The direct connection at their hind-ends between the cavities of the neural and alimentary canals is the most important of these. This is a result of the previous continuity of the epiblast and hypoblast at the point _x_, 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 alimentary canal is growing in such a way as to form the epithelium both of the dorsal and ventral surfaces of the fore-gut. 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 Bambeke. The points of agreement, though not obvious at first sight, I shall point out in the course of my description. The first stage (D I), 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 _Pelobates 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 normal with vertebrates, consists of a single layer of columnar cells.