STUDIES IN MAMMALIAN EMBRYOLOGY. 375 
pretation well) also in Vespertilio murinus. In all of 
these there is, ab origine, a layer of embryonic epiblast 
(trophoblast) which overcaps the germinal area before the 
formation of the amnion; the cavity that is thus overcapped 
becomes wholly or partially converted into the cavity of the 
amnion. We have only to suppose that a similar process takes 
place in man to understand that the retention of the epi- 
blastic connection between the embryonic region (germinal area) 
and the trophoblast at one point, would sufficiently explain 
the existence of a connecting stalk such as His’ “ Bauchstiel.” 
This stalk might then, indeed, be said to have been there from 
the beginning, the stalk-shape being substituted for the pre- 
vious circular connection in consequence of the formation of 
the amnion. 
If the stalk is situated, as it is in man, in the posterior 
prolongation of the axis of the embryo, we may infer that 
during the formation of the amnion folds, these folds have 
tended to coalesce, not in the middle above the embryo, but 
further backwards, the final “amnion-navel” then being 
applied against the ventral stalk without having taken any 
active or primary part in its formation, as Hertwig would 
have it. 
We must now consider in what way the mesoblast may have 
taken part in the formation of the ventral stalk. We have 
seen that the formation of the mesoblast (and I may add— 
though I have not adduced any proofs for this statement in 
this paper—its separation into a somatic and a splanchnic 
layer) takes place in the hedgehog, while the germinal area still 
retains its flattened curvature, and before any plastic indica- 
tion of the shaping of the embryo, much less any amnion fold, 
has become apparent (figs. 20 and 31). Similarly the area 
vasculosa is being formed during that early time, that the 
embryo is yet a convex surface, and before the line of the epi- 
blastic attachment, above referred to, has commenced its 
displacement upwards, from which the formation of the 
amnion results. Diagram fig. 31 illustrates this stage. In 
the posterior prolongation of the axis of the embryo we find in 
VOL, XXX, PART 3,——NEW SER. BB 
