8 CLASS MAMMALIA. 



the origin, and they consequently become the first conduits 

 of air to the lungs. The stomach of a young Opossum, 

 which weighed forty-one grains, was found considerably 

 distended and dilated, with a white and milky matter. 

 That of a younger one, on the contrary, contained a trans- 

 parent and colourless liquid. 



The eyes are opened after fifty or fifty-two days. The 

 teats are then quitted, and resumed successively. After 

 sixty days, a young opossum weighs 531 grains. Barton 

 relates a surprising circumstance of a female opossum, 

 having a double gestation of two separate litters, one draw- 

 ing to its close, and the other just commencing. This mo- 

 ther was then nursing seven young ones as large as rats. 

 Though strong enough to live on solid aliments, they still 

 had recourse to the teats for milk. But on a sudden the 

 pouch closed, for it had become the new domicile of seven 

 other young ones, weighing each from one to two grains. 

 Nevertheless, the first litter was not deprived of the cares 

 of the mother, who always manifested a constant attention 

 and affection for them all. Her watchfulness always ex- 

 tended to the family already brought up. She continued 

 the cry with which she was accustomed to call them back. 

 She assembled them on her back, and withdrew them, on 

 the appearance of danger, to the tops of trees. 



From all these facts, Barton in his first letter concludes 

 that these animals have two kinds of gestation : one which 

 he calls uterine, and which he considers to last from 

 twenty -two to twenty-six days ; and the other marsupial, 

 which commences from the entrance of the embryo into 

 the pouch. This last, physiologically speaking, is the 

 most important ; for the pouch, he adds, is strictly a se- 

 cond uterus, and the most important of the two. 



In the interval of the publication of his two letters, 

 Barton was informed that Sir Everard Home had published 

 a paper on the generation of the Kangaroos, in the Philo- 



