120 
and closely packed nuclei, but the protoplasm was not visibly divided 
into cell territories. This layer was thickest in the centre. It thinned 
towards the periphery and gradually disappeared. It was closely 
applied to the superjacent zona pellucida but was separated by a 
distinct boundary line from the subjacent layer (H Fig. 2). The latter, 
the inner layer of the germinal area consisted of oval nuclei sur- 
rounded by a granular protoplasm which formed only a thin layer on 
the surfaces of the nuclei but was accumulated in greater amount at 
their extremities. These nuclei were smaller and much less closely 
packed than the nuclei of the superficial layer of the area. They were 
situated at irregular distances from each other and the protoplasm 
which surrounded them also extended between them apparently in a 
continuous layer although when seen from the surface cell outlines 
were observed. 
When the inner layer was traced to the margin of the germinal 
area it was found to become continuous with the remainder of the 
cellular wall of the vesicle which in section appeared to consist of a 
number of flattened oval nuclei united together by a very thin layer 
of protoplasm. These nuclei and the protoplasm (H Fig. 1) which 
united them were essentially the same in character as the nuclei and 
protoplasm of the inner layer of the germinal area (H Fig. 2). 
Although the earlier and the later history of the two primitive 
layers of the ovum of the ferret is yet unknown there can be no 
doubt about their nature and ultimate fate, and a consideration of 
their relative positions and their special features induces the con- 
clusion that the outer is the epiblast and the inner the hypoblast. 
If the”inner layer is hypoblast then the remainder of the cellular 
wall with which it is continuous is also hypoblast and the blastocyst 
is essentially a large yolk sac which bears upon one pole a small 
area of epiblast. In which case the ovum of the ferret, as well as 
the ova of the rat and mouse, bears testimony to the descent of the 
mammalia from a large-yolked protamniote ancestry, for it shows 
during its early stages the typical features of all comparatively large- 
yolked vertebrate ova. 
In conclusion it is worthy of note, in connection with the question 
of the axial increase of the embryo which has latterly been again 
exciting attention, that there was no trace of any difference in thick- 
ness of the margin of the germinal area such as would indicate the 
existence of a widely opened true blastopore, nor was there any trace 
of such an aperture within the area itself. 
