382 ARTHUR ROBINSON. 
and consequently in these situations the foetal hypoblast is in 
direct contact with the maternal submucosa (42). 
At the commencement of the seventh day the original roof 
of the blastodermic cavity is separable into two parts: (1) an 
internal (#), oval in form, which is projected against and 
partially invaginates the central portion of the proximal wall 
of the hypoblast sac; and (2) an external (7’), which covers 
and extends beyond the internal, resting by its circular margin 
on the peripheral part of the proximal end of the hypoblast. 
The figures of ova at this stage given by Selenka and 
Duval differ somewhat from each other. Selenka figures and 
describes the inner mass of epiblast (Taf. i, fig. 44), but Duval 
sees no line of demarcation between the two, and he figures 
the inner mass and outer layer as directly continuous with 
each other (9, pl. i, fig. 83). Both Selenka and Duval de- 
scribe the lateral and distal walls of the yolk-sac as epiblast, 
the only hypoblast at this stage, according to their accounts, 
being that which constitutes the proximal wall of the yolk-sac 
and the irregular protoplasmic projections which extend from 
this mass towards the lateral and distal walls of the sac. The 
sac itself is, in their opinion, the remains of the cavity of the 
blastodermic vesicle, which only becomes converted into the 
yolk-sac by the extension of the hypoblast from the proximal 
end round the inner surface of the epiblastic wall, in a 
manner somewhat similar to that described in the rabbit by 
Van Beneden (2),in the mole by Heape (16), and in the shrew 
by Hubrecht (28). 
It has already been shown that the history of the ovum 
during the sixth day precludes such an interpretation of the 
appearances found at the commencement of the seventh day, 
and the following summary of important events of the former 
period will serve as a basis for the description of the changes 
which take place during the seventh day. 
