Now| UTERUS AND EMBRYO. 433 
folds rise up clear above the yolk. Moreover, the formation of 
the amnion is really a very complex process, part arising from 
the pro-amnion, part by a dilation of the pericardial cavity 
(Parietalhohle), and part as the extra-embryonic tail folds. 
These facts speak, in my judgment, unequivocally against the 
amnion having arisen by the sinking of the embryo into the 
yolk sack. Nor is there any justification, I think, for seeking 
these simple mechanical explanations, which are worthy of 
Herbert Spencer, since the formation of the amnion depends 
upon inequalities in the growth power of the germ layers, and 
only such explanation can be valid as explains that inequality — 
which Ryder’s hypothesis fails to do, so far as I can see. 
As regards the evolution of the placenta, we are in the dark. 
Contrary to prevalent opinion, it is not an organ of the allantois, 
nor is it an organ of the yolk sack. On the contrary, it is 
always, so far as we know, an organ of the chorion, and begins 
its development by a differentiation of that membrane. The 
allantois is a secondary and later structure. Its primitive réle — 
is apparently only that of a stalk of connection between the 
chorion and embryo. There is no evidence to show that the 
tissue of the allantois spreads out over the chorion to form the 
mesodermic layer thereof, but the mesoderm of the chorion is 
proper to it as much as to any part of the somatopleure the 
mesoderm thereof. When the allantois becomes a large sack, 
we have a subsidiary change, so that we are brought squarely to 
the conclusion that the foetal placenta is chorionic. From this 
premise phylogenetic speculation must start. Further, we know 
through the discovery of fundamental importance by His that 
the allantois cavity is at first a small entodermal tube lying ina 
posterior prolongation of the body (Bauchstzel), and that at this 
time the so-called allantoic vessels run to and branch out upon 
the chorion; the placental differentiation of the chorion has 
already begun, without participation of the allantois, the en- 
largement of which, when it occurs at all, occurs at a later 
stage. To speak, therefore, of an allantoic chorion as do Bal- 
four and Selenka (Studien iiber Entwichelungsges., p. 135) 1s 
unjustifiable. Nor can we trace the origin of the placenta to 
the yolk sack, since in most mammals the mesoderm does not 
spread over the yolk until quite late, so that the yolk sack 
consists, as in the rabbit and opossum, in large part of ectoderm 
