STUDIES IN MAMMALIAN EMBRYOLOGY. 373 
vesicle, the secondary chorion is the upper portion that over- 
caps it.” Of this hypothetical process he gives four woodcuts 
(l.¢., p. 171), and we will see that the hedgehog, while tending 
to confirm His’ first proposition, does not substantiate this 
latter hypothesis, but affords a more natural and at the same 
time more simple explanation. The outlines of this were 
already laid down by me in the ‘ Anat. Anzeiger,’ vol. ILI, 
p- 511; a more circumstantial development is, however, indis- 
pensable. 
We must first note that whereas Balfour remarks: ‘ His’ 
view has a good deal to be said for it;” onthe contrary, 
neither Kolliker nor Hertwig are inclined to accept the first 
proposition here alluded to. According to Kolliker (‘ Ent- 
wickelungsgesch. des Menschen,’ 2e Aufl., p. 367) the human 
allantois grows out of the embryo in the vesicular form only 
just the distance till it has reached the serous membrane. 
When this has come about it is only its connective tissue-layer 
with the blood-vessels, which further spreads over the whole 
inner surface of the serous membrane. And so it is the bud- 
ding allantois by which the ventral stalk is formed. It is this 
interpretation against which, as we have seen, His brings 
forward very weighty objections; for many of which we must 
refer the reader to the original treatises. 
Hertwig (‘ Lehrbuch der Entwickelungsgesch.,’ le Aufl., p. 
181) tries to reconciliate Kolliker’s and His’ interpretations, and 
attempts to fill up the blanks between those developmental 
stages of the ventral stalk, which have as yet come to our 
knowledge by the assumption : 
(1) that at the formation of the human amnion it never 
wholly separates from the serous envelope, but remains con- 
nected with it along a drawn-out posterior prolongation; 
(2) that the ventral stalk is then formed by an allantoidean 
projection budding forth out of the embryo, and travelling 
towards the serous membrane along this pre-existent con- 
nection. 
The words in which Hertwig couches this hypothetical inter- 
pretation are the following: 
