Sir Everard Home on the placenta. 403 
In 160 cows, some calved in 241 days ; 5 in 308 ; giving 
a latitude of 67 days 
In 102 mares, 3 foaled in 311 days ; 1 in 394 days ; giving 
a latitude of 83 days. 
In 15 sows, 1 littered in 109 days ; 1 in 123 days; giving 
a latitude of 14 days. 
In 139 rabbits, 1 produced its young in 26 days ; 9 in 33 
days ; giving a latitude of 7 days. 
As the human species has the form of placenta best fitted 
for the supply of blood to the foetus, there appears to be less 
latitude than in other animals, at least in civilized society, where 
the nourishment of the mother is an object of the first atten- 
tion ; it may be otherwise in women living in a savage state. 
Where the female of one species of animals breeds from 
the male of another, the utero-gestation of whose species is 
different, there appears to be no approximation in the time 
the hybrid is brought forth ; but the longest period of the 
two is the time of such utero-gestation. 
The mare, when covered by an ass, goes 11 months, her 
usual period. The Earl of Morton's mare, covered by the 
quagga, went 339 days 19 hours. 
The she ass, when covered by a horse, goes 11 months, 
although ten is her usual period. 
The direct cause of parturition has never been satisfactorily 
explained ; and the great latitude there is in the utero-gesta- 
tion of individuals of the same species, which has just been 
shown, makes it evident, that the cause must be something 
immediately connected with the complete formation of the 
foetus. Upon this subject I am induced to hazard the fol- 
lowing observations. 
