EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES, 21] 
between the two poles is apparent at the corresponding 
period (Plate X, fig. a 1). 
The segmentation cavity in this ovum is not quite so large 
proportionately as in Amphioxus, and the encroachment upon 
it is due to the larger bulk of the lower pole of the egg. In fig. 
B 11, the involution of the lower pole has already com- 
menced ; this involution is (1) not quite symmetrical, and (2) 
on the ventral side (the left side) the epiblast cells forming the 
upper part of the egg are growing round the cells of the 
lower pole of the egg or lower-layer cells. Both of these 
peculiarities are founded upon what happens in the Frog and 
the Selachian, but it is to be noticed that the change from 
the lower layer cells being involuted towards the epiblast 
cells, to the epiblast cells growing round the lower layer 
cells, is a necessary consequence of the increased bulk of the 
latter. 
In this involution not only are the cells of the lower pole 
pushed on, but also some of those of the upper or yellow 
portion ; so that in this as in all other cases the true dis- 
tinction between the epiblast and hypoblast does not appear 
till the involution to form the latter is completed. In the next 
stage, B 111, the involution has become nearly completed and 
the opening to the exterior or Blastopore quite constricted. 
The segmentation cavity has been entirely obliterated, as 
would have been found to be the case with Amphioxus 
had the stage a little older than that on Plate X, a 111, been 
represented. ‘The cavity marked (a /), as was the case with 
amphioxus, is that of the alimentary canal. 
The similarities between the mode of formation of the 
hypoblast and alimentary canalin this animal and in Amphi- 
oxus are so striking and the differences between the two cases 
so slight that no further elucidation is required. One or two 
points need to be spoken of in order to illustrate what occurs 
in the Frog. When the involution to form the alimentary canal 
occurs, certain of the lower layer cells (marked h y) become 
distinguished from the remainder of the lower layer cells as a 
separate layer and form the hypoblast which lines the ali- 
mentary canal. It is to be noticed that the cells which form 
the ventral epithelium of the alimentary canal are not so 
soon to be distinguished from the other lower layer cells as 
those which form its dorsal epithelium. This is probably a 
consequence of the more active growth, indicated by the 
asymmetry of the involution, on the dorsal side, and is a 
fact with important bearings in the ova with more food 
material. The cells marked mand coloured red also become 
distinguished as a separate layer from the remainder of the 
