MARSUPIALIA. 
dent half a line below the origin of the pedicle. 
(See the foetus in fig. 141.) 
The small intestine above the pedicle was 
disposed in five folds. The first from the 
stomach or duodenum curved over the vitel- 
line vein, and the remaining folds were dis- 
posed around both the vitelline vessels. From 
the cecum, which was given off from the re- 
turning portion of the umbilical loop of the 
intestine, the large intestine passed backwards 
to the spine, and was then bent, at a right angle, 
togo straight down to the anus. The stomach did 
not present any appearance of the sacculated 
structure so remarkable in the adult, but had 
the simple form of a carnivorous stomach. 
The liver consisted of two equal and symme- 
trically disposed lobes. The vena porte was 
formed by the union of the vitelline with the 
mesenteric, and doubtless the other usual veins, 
which were, however, too small to be distinctly 
perceived. The diaphragm was perfectly 
formed. 
The vena cava inferior was joined, above 
the diaphragm, by the left superior cava, just 
at its termination in a large right auricle. The 
ventricles of the heart were completely joined 
together, and bore the same proportions to each 
other as in the adult,—a perfection of structure 
which is not observed in the embryos of ordi- 
nary Mammalia at a corresponding period of 
development. The pulmonary artery and 
aorta were of nearly the same proportionate 
size as in the adult: the divisions of the pul- 
monary artery to the lungs were at least double 
the size of those observable in the embryo of a 
sheep three inches in length. The ductus 
arteriosus, on the contrary, was remarkably 
small. The aorta, prior to forming the de- 
scending trunk, dilated intoa bulb, from which 
te carotid and subclavian arteries were given 
off. 
The lungs were of equal size with the heart, 
being about a line in length, and nearly the 
same in breadth: they were of a spongy tex- 
ture and of a red colour, like the veins, from 
the quantity of blood they contained. This 
precocious development of the thoracic viscera 
is an evident provision for the early or prema- 
ture exercise of the lungs as respiratory organs 
in this animal: and on account of the simple 
condition of the alimentary canal, the chest 
at this period exceeds the abdomen in size. 
The kidneys had the same form and situ- 
ation as in the adult. The supra-renal glands 
were half the size of the kidneys. 
The testes were situated below the kidneys, 
and were one-half larger than those glands, the 
superiority of size depending on their large 
epididymis, with the adherent remains of the 
Wolffian body. They continue within the ab- 
domen for six weeks after uterine birth. 
At a later period of uterine development, 
when the fetus, measured in a straight line 
from the mouth to the root of the tail, is ten 
lines in length, the urachus expands into a small 
allantois (d, fig. 141), of a flattened pyriform 
figure,and finely wrinkled external surface. This 
bag insinuates itself between the amnios and 
chorion, carrying along with it two small hypo- 
325 
gastric arteries and an umbilical vein, but not 
establishing by their means an organized and 
vascular surface of the chorion by which a 
placental attachment is formed between the 
ovum and the womb. The allantois depends 
freely from the end of the umbilical chord, 
and has no connexion at any part of its cir- 
cumference with the adjoining membrane. Its 
office is apparently that of a receptacle of 
urine. 
The vitellicle or umbilical sac presented the 
same large proportionate size and vascular 
structure as in the first described foetus. The 
chorion which enveloped this foetus and its 
appended sacs was adapted to the cavity of the 
uterus by being disposed in innumerable folds 
and wrinkles. It did not adhere at any part 
of its surface to the uterus, but presented a 
modification not present in the chorion of the 
earlier foetus, in being partially organized by the 
extension of the omphalo-mesenteric vessels 
upon it from the adherent vitellicle. The di- 
gits of the hind legs were distinctly formed in 
this embryo. 
The new-born foetus of the great Kangaroo 
does not exceed, as we have already shown, 
one inch in length; its external characters have 
been already described. Dr. Barton has given 
the following account of the Opossum ( Didel- 
phys Virginiana) at an analogous period. 
*¢ I have been so fortunate as to ascertain the 
size and weight of several embryos imme- 
diately after their exclusion from the uterus. 
One of them weighed only one grain! The 
weight of each of the six other young ones 
was but little more than this. The young 
opossums, unformed and perfectly sightless as 
they are at this period, find their way to the 
teats by the power of an invariable, a deter- 
minate instinct” (qu.?), In this new domi- 
cilium they continue for about fifty days, that 
is, until they attain the size of a common 
mouse (Mus musculus), when they begin to 
leave the teats occasionally, but return to them 
again until they are nearly the size of rats. 
“ At the end of about fifty or fifty-two days 
from its first reception in the pouch the eyes of 
the young begin to open. 
“ T have found that the same embryon has 
increased in weight 531 grains in sixty days, 
that is, at a rate of almost 9 grains daily. The 
animal attains to nearly its full growth in about 
five months; but never, I believe, (in our lati- 
tudes 1 mean,) procreates the first year of its 
existence. : 
-“ On the 21st of May, upon looking into 
the box which contained the female Opossum, 
I found that she had just excluded from her 
uterus seven embryons; the smallest of which 
scarcely weighed one grain, another barely two 
grains, and the remaining five (taken together) 
exactly seven grains.” * 
In the Kangaroo about ten months elapse 
before the mammary fetus quits the pouch: 
it has, prior to this period, quitted the nipple, 
* Barton, in Annals of Philosophy, vi. (1823), 
p. 349. <« Facts, Observations, and Conjectures relative 
to the Generation of the Opossum.” 
