120 EARLY STAGES IN THE 



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 Pelobatcs 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 l), 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 Pelobatcs 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. 



