536 PHILOSOPHICAL TRANSACTIONS. [aNNO 17Q5. 



ance, till it acquires a perfect form, has been very accurately traced ; but this 

 may be considered as more properly belonging to the economy of the young, 

 than to the history of generation itself. 



The opossum tribe, wliich the kanguroo resembles in the structure of its ge- 

 nerative parts, differ in the economy of their young from other quadrupeds ; and 

 as it is found that this difference is an approach towards the economy of animals 

 of another class, the descriptions and observations which are now to be given will 

 be better understood by stating, in general terms, the different modes employed 

 by nature for supporting the young till it is enabled to receive food by the 

 mouth. 



In quadrupeds in general, the ovum containing the embryo, as soon as it 

 arrives in the uterus, becomes attached to the internal surface, and the foetus 

 owes its increase and support to a connection with that viscus, by ineans of the 

 placenta and navel string. In the bird, the snake, the lizard, the tortoise, and 

 in fish, the nidus of the embryo, even before its impregnation, is detached from 

 the mother, and the foetus receives its future support from the animal substance 

 in which it is enveloped. In some of these, the egg which contains the young 

 is deposited in the oviduct of the mother, and there hatched ; in others it leaves 

 the oviduct altogether, and is hatched out of the body; but in all cases of detached 

 foetuses, before the young leaves the shell, the remaining contents of the egg 

 pass up into the belly, which is immediately closed after it comes into the air, 

 and therefore there is no appearance of external connection similar to the navel 

 in quadrupeds. 



In the following account, the foetus of the opossum tribe will be found neither 

 to derive its support from a connection with the uterus in which it is deposited, 

 like other quadrupeds, nor exactly to resemble in the mode of its nourishment 

 the young that is hatched from an egg, but to have a mode of support peculiar 

 to itself. It therefore appears to form a link in the gradation leading from the 

 one to the other. The American opossum, which is a small animal, was the 

 only one of this tribe that was known in Europe before the late discoveries in the 

 South Seas , and as it had not been found to breed either in France or England, 

 the only accounts of its mode of generation were these received from America, 

 which were vague, and could not be entirely depended on. These accounts how- 

 ever led anatomists, who had opportunities of dissecting the female organs, to 

 endeavour by that method to throw some light on the subject ; but the parts 

 were found to be so complex, and in so many respects different from those of 

 other quadrupeds, that nothing satisfactory could thus be made out, while de- 

 prived of an opportunity of seeing them in an impregnated state.* 



* In Bufton's Histoire Naturelle there is an anatomical description of the female organs of tlie 

 opossum^ by Daubentonj and quotations from an account published in England by I'yson, from 



