434 MINOT. [Vot. II. 
and entoderm only, and is without vessels, and therefore unable 
to form a placenta, which, however, is developing meanwhile 
from the chorion. 
We seek nowadays, following the lead of Professor Cope, to 
deduce mammalia from the reptilia. Since the reptilia have a 
free allantois, it is a temptation for embryologists to seek to 
trace the placenta to a modification of the allantois; but the 
placenta of mammals appears in the embryo before the allantois 
becomes free, and the great size of the allantoic vessels is con- 
nected primitively not with the allantois, but with the already 
important chorionic circulation. The placenta is interpolated 
in the ontogeny of mammals before the specialization of the 
allantois, which functions as the vascular pathway between the 
embryo and the chorion, both primitively and permanently. 
The enlargement of the allantois, which takes place in certain 
mammals, is a supervening change, probably a survival of rep- 
tilian ontogeny. The question is, not how is the connection of 
the allantois with the placenta (chorion) established in mammals, 
for it exists from the start,! but what becomes of it in reptiles 
and birds. 
Ryder’s theory, 128a, of the origin of the discoidal pla- 
centa? by constriction of the villous area of the zonary placenta, 
is difficult to accept. The placenta, being chorionic, cannot of 
course develop, except so far as the chorion is differentiated ; 
that is to say, so far as the ectoderm (exochorion) is underlaid 
by mesoderm. Now, in mammals, the chorion, as mentioned 
above, does not go at first but part way over the yolk sack, even 
at the period when the development of the placenta has begun. 
Accordingly, so far as our present knowledge enables us to 
judge, the discoidal is probably the primitive placental type. 
If the chorion is completed by the further extension of the 
mesoderm around the yolk sack, then the placental formation 
also may spread, and a diffuse type arise. At present, the 
whole subject is very obscure, but there is certainly no suffi- 
cient evidence to prove that the diffuse placenta is the primi- 
tive type. 
In conclusion, let me point out that we have no satisfactory 
1 This is beautifully shown by Selenka’s investigations on the opossum, cited in the 
text. 
2 The human placenta is of discoidal, but metadiscoidal. 
