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necting the embryo with chorion; in this type the allantois brings the 
blood vessels to the chorion, and the vessels then ramify over the 
chorion itself, which has therefore its own circulation though dependent 
upon the allantois; this modification is characteristic of the unguiculate 
series of mammals. 2° The allantois grows out into a large vesicle, 
which has for some time no connection with the chorion but maintains 
a well developed circulation of its own; its expansion brings it ulti- 
mately into contact with the chorion and its outer or mesodermic 
layer grows together with the inner or mesodermic layer ofthe chorion 
(Bonner [2]), which thus becomes indirectly vascularized; this modi- 
fication is characteristic of the ungulate series of mammals. How far 
other modification distinct from these may exist we cannot say at 
present. 
We have then two types: 1° the chorion has its own vessels (un- 
guiculates); 2° the chorion acquires vessels by growing together with 
the vascular walls of the allantoic vesicle (ungulates). 
In both cases the chorion is the part of the foetus and the only 
part in direct contact with the uterine wall, and therefore in both 
cases it is the essential part of the foetal placenta. In unguiculates 
the chorion after it receives its blood vessels has its own blood supply 
and circulation, and therefore suffices to develope the placenta. In 
ungulates the circulation is entirely allantoic, and the walls of the 
allantois are essential to maintain the foetal circulation of the pla- 
centa; the chorion therefore does not suffice to develope the foetal 
placenta. While therefore we recognize that the chorion is always 
the means of union between the mother and the offspring we may 
conveniently distinguish the unguiculate type as having a true chori- 
onic placenta and the ungulate type as having an allantoic 
placenta. 
Evolution of the Placenta. As regards the evolution of 
the placenta, our conceptions are still very obscure. The opinion was 
long, and perhaps still is, generally prevalent that the placenta is 
primarily an organ of the allantois. This notion was one of those 
theories, which sometimes become current, without ever having been 
supported by adequate proof, and are repeated until tradition has 
rendered them venerable, and age gives them a dignity, which their 
worth does not entitle them to. The principal support of this theory 
was derived from the fact that the allantois is connected with the 
placental circulation. Batrour in 1881 (Works I, 743) sought to 
modify this view by attributing importance to the relations of the 
yolk sack, which he believed to be the means of maintaining the cir- 
