378 A. A. W. HUBRECHT. 
deciduo reflexa both in the hedgehog and in man affords very 
weighty arguments—we may safely say that this “enucleating” 
of the human yolk-sac out of the somatic mesoblast and tropho- 
blast G.e. out of the diplotrophoblast) may occur at any stage 
of development, even the very earliest. 
Thus the many curious abnormalities of the early human blasto- 
cyst may one by one be viewed in a simple light now that we have 
the hedgehog to guide us. His, when he wrote the pages 
above referred to, was much more groping in the dark; still it 
seems to me that he was very happy in many of his conjectures, 
and that, at all events, he was right in assuming that the 
“ventral stalk” is there ab origine. Hertwig’s view, accord- 
ing to which the ventral stalk is ulterior to the origin of the 
amnion folds, is sufficiently refuted in the foregoing pages. 
And for the second portion of His’ hypothesis, which led him 
to the distinction of a “ primary ” and ‘‘ secondary ” chorion, 
we have already interpolated the simpler explanation which 
immediately offers itself if we assume that in man a layer of 
trophoblast, such as we find it in the hedgehog and other 
mammals, overcaps the embryonic area (cf. figs. 16 to 20), and 
that the process of the formation of the amnion follows similar 
lines as those traced for the hedgehog (fig. 51). 
We must now turn our attention for a few moments to the 
chorion of the human blastocyst and to the part it plays in 
placentation. 
The earliest human blastocysts that have been investigated 
have always shown a remarkable villosity. These villi were 
examined by numerous authors. Only lately Sedgwick Minot 
has given detailed descriptions and numerous woodcuts of 
them (‘ Journ. of Morphology,’ vol. ii, No. 3). The presence 
of villi was noticed by Coste, even before any mesoblast 
contributes towards their formation; they are said to be 
hollow, and the vascular mesoblast was then noticed to spread 
rapidly against the epiblast and to fill up these hollow recesses. 
The wholevilliferous blastocyst is said to rest loosely in the closed 
cavity formed by the decidua reflexa (Kolliker, Entwickelungs- 
gesch. des Menschen, 2e Aufl., p. 374); the fixation of the 
