32 
SYSTEM OF NATURE. 
deposited in the marsupium or uterus. Now the egg or 
embryo of the Mammalia has the property of attaching 
itself to every part of the uterus at the point where the 
placenta is formed; and thus the embryo or egg of the 
marsupials fastens itself to the mamma, and there commu¬ 
nicates with the arteria epigastrica, in the same manner as 
in other Mammalia it communicates with the arteria ute- 
rina. It is fastened by a cord resembling the navel-string, 
(though it is unknown where this cord passes out from the 
embryo), which is often so long that the embryo hangs 
out of the bag, and which, at the moment of real birth, is 
separated by a rupture, as in the case of the placenta and 
ordinary uterus. This external uterus, however, does not 
invariably assume the form of a purse or bag; in some in¬ 
stances it consists of simple folds of the skin, and in the 
monotremes even these disappear. 
“ The monotremes bear a very strong affinity to the or¬ 
dinary marsupials : they likewise very closely resemble 
birds , not alone in the construction of the bill , cranium , 
clavicles , shoulder-bones , sternum , and undeveloped teats , 
but especially in their organs of generation. These ani¬ 
mals have only one ovarium developed, as in birds, and 
both the Echidna and the Ornithorhynchus lay eggs and 
hatch them. Thus it is that the uterus of the Mammalia 
becomes modified in the marsupials, so as to be situated 
without the body, and finally vanishes altogether in the 
Monotremata. 
“ If we apply these considerations to ascertain the con¬ 
catenation of the various groups of animals, in relation to 
their organs of generation, we find that it indicates one 
class, the Mammalia, which have an internal hatching 
organ, called the uterus ; another class, the marsupials and 
