78 ANIMAL PHYSIOLOGY. 
blast only, forming a cavity into which the anterior part of the 
embryo early projects (pro-amnion). 
The true amnion arises only from the posterior end of the 
embryo, and, extending over in a forward direction, meets the 
raised projection of the pro-amnion with which it fuses. 
The amniotic cavity becomes one with that space (extra-em- 
bryonic pleuro-peritoneal cavity) arising from the cleavage of 
Fie, 86.—Fcetal parolopes of a rabbit embryo (Minot, after Van Beneden and Julin). Later 
mints than Fig. 8 B. The amnion has become fused with the blastoderm in front of the 
embryo, and its cavity is therefore continuous with the extra-embryonic portion of the 
body-cavity in front of the embryo. Al, allantois; am, amnion; am’, portion of the 
amnion united with the walls of the allantois ; 4. pl, area placentalis ; Av, area vasculosa ; 
Ch, chorion; Cae, ccelom or body-cavity ; Cae’’, extra-embryonic portion of the body- 
. cavity ; Col, anterior portion of the same, produced by the fusion of the cavity of the 
amnion with that of the anterior portion of the area opaca; Ec, epiblast ; En, alimentary 
canal of the embryo; Ent, hypoblast ; Pl, placenta; pro..A, proamnion; 7, sinus ter- 
minalis ; V, villi of blastodermic vesicle ; Y, cavity of blastodermic vesicle. 
the mesoblast, which now advances beyond the head of the em- 
bryo and the pro-amnion. The pro-amnion by gradual atrophy 
gives place to the true amnion. , 
At about the same period as these events are transpiring the 
vascular yelk-sac has become smaller, and the allantois with its 
abundant supply of blood-vessels is becoming more prominent, 
and extending between the amnion and subzonal membrane. 
The formation of the chorion marks an important step in 
the development of mammals in which it plays an important 
functional part. It is the result of the fusion of the allantois, 
which is highly vascular, with the subzonal membrane, the villi 
of which now become themselves vascular and more complex 
in other respects. 
An interesting resemblance to birds has been observed (by 
Osborn) in the opossum. When the allantois is small the 
