VIVIPAROUS ANIMALS. 403 
fluid exhibits very different properties, according to the 
species from which it is obtained, The skin of the foetus 
is frequently covered with a fatty matter, which some con- 
sider as a deposition from the liquor amnii, but which 
others, with greater propriety, regard as a cutaneous se- 
cretion. Indeed, suspicions have arisen that the liquor it- 
self is secreted by the foetus; and these are countenanced 
by the circumstance, that, in the amnios of some species, as 
man, no bloodvessels can be traced in its structure. What- 
ever may be its origin, it is obviously useful im protecting — 
the foetus from being injured by any sudden shock, or com- 
pression, , 
The chorion, inmany animals, serves the purpose of a sup- 
port to the vessels which form the umbilical cord. It joins 
the amnios at the umbilical cord, and is united with the uterus 
wherever there is a placenta. In the sow and mare it is 
united to the whole internal surface of the uterus by nume- 
rous tubercles. In the ruminating quadrupeds these tu- 
bercles appear to collect in groups, which have been termed 
cotyledons. 'These consist of the glandula uterine or fleshy 
excrescence of the inner surface of the uterus, and the ca- 
runcule or corresponding glands of the chorion. In the 
sheep and goat the glandule are concave, and receive the 
convex surface of the carunculee, while this arrangement is 
reversed in the cow and deer. The third kind of placenta 
may be considered as arising from the more complete union 
of the tubercles, not into cotyledons, but into two masses, 
sometimes only one, either lying close together, or spread 
out like a belt. 
In many quadrupeds there is a peculiar sac, termed 
allantois, which occurs between the amnios and chorion in 
some cases, as the mare, occupying the whole cavity; in 
others, as the sheep and cow, it is more limited im its ex- 
cca 
