VIVIPAROUS ANIMALS. 411 



pregnation. In whatever manner the determination of the 

 sexes is effected, we perceive, that, among the number ac- 

 tually produced, the males and females bear such a rela- 

 tive proportion, as to secure a continuance of the race. 



The time which elapses between the sexual union, and 

 consequent impregnation of the ovum, to the perfection of 

 the foetus and its expulsion by birth, differs greatly accord- 

 ing to the species, and does not appear to be regulated by 

 any peculiar systems of organization. The term of gesta- 

 tion in the lionness and sheep is the same, or five months ; 

 the horse and the ass, animals of the same genus, have si- 

 milar terms, eleven months ; while the cow and the buffalo, 

 likewise belonging to one genus, have dissimilar terms, the 

 former going with young nine months, while the gestation 

 of the latter occupies twelve. Even different individuals of 

 the same species vary in this respect a few days, and even 

 the same individual at different times. 



At birth, the foetus is expelled the uterus, the connec- 

 tion with the placenta ceases by the division of the umbili- 

 cal cord, the enveloping membranes are torn asunder, and 

 the young animal leaves its watery dwelling to enjoy a 

 more independent existence. The blood from the heart, 

 now interrupted in its exit from the system through the 

 umbilical arteries, is transmitted in quantity to the lungs, 

 the thorax is thereby excited to action, and the important 

 function of respiration commences. The bloodvessels which 

 were necessary to carry on the modified circulation with the 

 placenta, together with the ductus arteriosus, and urachus, 

 change their nature, and are converted into ligaments. The 

 foramen ovale likewise closes, unless in some of those qua- 



by one father, in Ireland, and who all arrived at the age of manhood ; and 

 Mr KIRWAN often declared, he had no reason to doubt the truth of this rela- 

 tion," 



