222 Mr. Home's Observations on the 
The opossum tribe, which the kanguroo resembles in the 
structure of its generative parts, differ in the oeconomy of their 
young from other quadrupeds ; and as it will be found that this 
difference is an approach towards the oeconomy 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 means 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 de- 
tached from the mother, and the foetus receives its future sup- 
port from the animal substance in which it is enveloped. 
In some of these, the egg which contains the young is depo- 
sited 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 
