EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES, 213 
is only separated by a slit from the remainder of the lower 
layer cells; but by the stage represented in c 11 this has 
widened into an elongated cavity (a 7). In its formation 
this involution pushes backwards the segmentation cavity, 
which finally disappears in the stage c 111. ‘The point 2 
remains practically stationary, but by the general growth of 
the epiblast, mesoblast, and hypoblast, becomes further re- 
moved from the segmentation cavity inc 1rthanincr. On 
the opposite side of the embryo to that at which the involu- 
tion occurs the epiblast cells as before, grow round the lower 
layer cells. ‘lhe commencement of this is already apparent 
in C1, and inc 11 the process is nearly completed, though 
there is still a small mass of yolk filling up the blastopore, 
The features of this involution are in the main exaggerations 
of what was supposed to occur in the previous animal, The 
asymmetry of the involution is so great that it is completely 
one-sided and results, in the first instance, in a mere slit; and 
the whole process of enclosing the yolk by epiblast is effected 
by the epiblast cells on the side of the egg opposite to the 
involution. 
The true mesoblast and hypoblast are formed precisely as 
in the previous case. The involuted cells become separated 
into two layers, one forming the dorsal epithelium of the 
alimentary canal, and a layer between this and the epiblast 
forming the mesoblast. There is also a layer of mesoblast 
accompanying the epiblast which encloses the yolk, which is 
derived from the smaller green cells at y (c1). The edge 
of this mesoblast, m’, forms a thickened ridge, a feature which 
persists in other vertebrates. 
It isa point of some importance for understanding the 
relation between the mode of formation of the alimentary 
canal in the frog and other vertebrates to notice that on the 
ventral surface the cells which are to form the epithelium of the 
alimentary canal become distinguished as such very much later 
than do those to form its dorsal epithelium, and are derived 
not from the involuted cells but from the primitive large yolk- 
cells. It is indeed probable that only a very small portion 
of epithelium of the ventral wall of the mid-gut is in the end 
derived from these larger yolk-cells. The remainder of the 
yolk-cells (c 111, and c 11, yk) form the yolk mass and 
do not become directly formed into the tissues of the 
animal. 
In the last stage I have represented for the frog, c 111, there 
are several features to be noticed. 
The direct connection at their hind-ends between the 
