402 Sir Everard Home on the placenta. 
terials for the growth of its different parts, and when we 
find so great a variety in the structure of placentas in dif- 
ferent animals, it seems to follow that every genus of animals, 
in as much as it has a peculiar form, should have a peculiar 
placenta ; and this, upon examination, is found to be the case. 
The difference in the form of the placenta, which has hi- 
therto had so little attention paid to it, is therefore to be con- 
sidered as the means employed by nature to prevent the 
whole system respecting animals from being thrown into con- 
fusion, by preventing any two different genera from breeding 
together. We now see why this cannot take place, since a 
new form of placenta would be required intermediate between 
those of the two genera, for which there is no provision. 
Upon the structure of the placenta or chorion must depend 
the period of utero-gestation. Where they are very vas- 
cular, it will be short, and where the reverse, very long. The 
human placenta is massy, thick, its arteries very large, and 
numerous ; the utero-gestation is nine months. The pla- 
centa is wanting in the mare, there being only a very vas- 
cular chorion ; the utero-gestation in that genus is eleven 
months. The utero-gestation of the elephant, according to 
Mr. Corse, is twenty-two months ; from which I am led to 
conclude there is no placenta. 
The placenta would appear to be more or less in a perfect 
state, according to the care which is taken of the animal. 
The cow's utero-gestation at a mean is 284 days. The wild 
cow is stated to go 308, the longest period I believe re- 
specting cows upon record. 
This explains the latitude met with in utero-gestation, 
which is noticed in the Bulletin de Sciences, by the Philo- 
matique Society in Paris, for the year 1 797, by Mr. Tessier. 
