380 | ARTHUR ROBINSON. 
The depression at the proximal pole (P) probably represents 
the remains of the invagination cavity, and from the bottom 
of it a dark line is traceable for a short distance into the inner 
cell mass, which does not contain a cavity. 
The diminution in height of this ovum as compared with the 
specimen represented in fig. 6, Pl. XXIII, is due to the projec- 
tion of the inner cell mass into the protoplasm of the distal 
pole of the ovum, and by the same event the segmentation 
cavity (S C) has been transformed into a circular canal which 
encircles the proximal end of the inner cell mass. A second 
circular canal (V C) is seen, distal to the modified blasto- 
dermic cavity. It is situated in the protoplasmic mass which 
forms the floor of the blastodermic vesicle. It is the rudiment 
of the yolk-sac. 
The protoplasm of the distal pole of the ovum is apparently 
dividing into two layers, for a line of demarcation is already 
visible on the section, extending between the two segments of 
the yolk-sac through the distal protoplasmic mass. 
It is during the sixth day, therefore, that the various com- 
ponent parts of the blastodermic vesicle present, for the first 
time, clear indications of their nature and ultimate fate. 
The thick faintly staining floor of the early blas- 
todermic vesicle is the distal, not the proximal wall. 
It does not divide into an outer layer of epiblast 
and an inner of hypoblast, but in it a cavity develops, 
and it becomes the hypoblastic lining of the alimen- 
tary canal and yolk-sac of the embryo. 
The thin roof of the young blastodermic vesicle is 
the proximal, not the distal wall. A portion of it is 
invaginated into the cavity of the vesicle, thus pro- 
ducing the inner cell mass. A portion of it ulti- 
mately becomes the embryonic epiblast, and the 
remainder takes part in the formation of the pla- 
centa. 
The comparison which Duval makes between the early 
stages of the ovum of the mouse and that of the rabbit cannot 
be sustained, for before the sixth day there is no inner cell 
