326 
and occasionally protrudes its head and changes 
its position in the pouch. 
anatomical condition and progressive 
development of the rm foetus of the 
Marsupialia offer a subject of highly interest- 
ing research, especially if com with the 
same circumstances in the uterine feetus of an 
se sized and analogous placental aa 
uch still remains to be done in this chapter 
of the history of Marsupial generation; at 
present J have to offer the following obser- 
vations, 
By comparing the new-born Kangaroo with 
a similarly sized foetus of a sheep, we find that, 
although, in the Kangaroo, the ordinary laws 
of development have been adhered to in the 
more advanced condition of the anterior part 
of the body and corresponding extremities, yet 
that the brain does not present so dispropor- 
tionate a size; and the same difference is ob- 
servable in the uterine fcetus of the Kangaroo, 
even when compared with the same sized em- 
bryo of an animal of an inferior class, as the 
bird. This difference, I apprehend, is owing 
to the rapidity with which the heart and lungs 
acquire their adult structure in the Kangaroo, 
whereby the passage of the purer and more 
nutritious blood through the foramen ovale and 
left auricle to the primary branches of the 
aorta and so to the brain is impeded. The 
brain, however, of the mammary fetus, though 
exhibiting a low degree of development, yet is 
of a firmer texture than in a similarly sized 
foetus of a sheep, and attains its ultimate pro- 
portion by a more gradual process of growth. 
In a mammary fetus, one inch and a half 
in length, the urinary bladder is largely deve- 
loped, and adheres by its apex to the perito- 
neum, exactly opposite that part of the abdo- 
minal integument where a small linear ridge 
indicated the previous attachment to the umbi- 
lical chord and appendage. There are also 
minute but distinct traces of umbilical arteries 
running up the sides of the bladder to this point 
of attachment. As the urinary bladder be- 
comes afterwards expanded in the abdomen, 
the peritoneum is gradually, as it were, drawn 
from this part of Ke wudowinal ietes, form- 
ing an anterior ligament of the bladder. Ina 
mammary fetus of the Kangaroo about a month 
older than the above, there was at the superior 
= of this duplicature a small projecting point 
rom the bladder, like the remains of a ura- 
chus; but the fundus, now developed con- 
siderably above this point, was covered with 
a perfectly smooth layer of peritoneum; and 
it is this modification, I apprehend, which 
led Hunter to suppose that there was no trace 
of urachus or umbilical arteries in the fetuses 
of the Marsupialia. In the Sloth, the Manis, 
and the Armadillo, the urachus is continued 
in the same manner from the middle of the 
anterior part of the bladder, and not from the 
fundus. 
In neither of the above foetuses of the Kan- 
garoo was there any corresponding trace of 
umbilical vein, although there was a distinct 
ligamentum suspensorium hepatis, formed by 
a duplicature of the peritoneum descending 
MARSUPIALIA. 
from the diaphragm to the notch lodging the 
gall-bladder, and not entering, as usual, th 
fissure to the left of that notch: the allantois is” 
too small, and its function too limited for the 
preservation of any permanent trace of its” 
peculiar vein. NES 
The small intestines in the mammary foetus, 
one inch and a half long, when compared 
with those of the uterine fetus above de- 
scribed, were found to have acquired several 
additional convolutions ; the fold to which the 
umbilical vesicle had been attached was still — 
distinct, but now drawn in to the back of the 
abdomen. The cecum was much elongated, — 
but the colon proportionately not more deve- 
loped than in the uterine fetus; the subse- 
quent modification, therefore, of the large in- 
testines seems evidently destined to com 
the digestion of the vegetable food. t 
The stomach was not sacculated, but the — 
division between the cardiac and middle com- — 
rtments was more marked than in the uterine 
cetus. The liver had now advanced in its 
development beyond the oviparous form which — 
it presented in the uterine fetus, the right lobe © 
being subdivided into three. The su L 
glands bore the same ionate size to the 
kidneys. The testes were still larger than the 
kidneys, and were situated below them, not — 
having yet passed out of the abdomen: this 
takes place when the mammary feetus is about — 
three inches long from the nose to the root of 
the tail. The ductus arteriosus was distinct in 
the small mammary feetus, but I could i 
perceive any trace of the thymus gland. Is — 
this gland unn on account of the pre~— 
cocious development of the lungs? or because 
of the small size and gradual growth of the 
brain? The latter appears the more probable — 
condition of its absence, as in the ovovivipa- 
rous classes with small and simple brains — 
the thymus gland is rudimental or of doubtful 
existence. ita 
Notwithstanding that the new-born Kanga- — 
roo possesses greater powers of action than the — 
same sized embryo of a sheep, and 
mates more nearly in this respect to the ne 
born young of the rat, yet it is evidently in- 
ferior to the latter. For, although it is enabled 
by the muscular power of its lips to grasp and 
adhere firmly to the nipple, it seems to be 
unable to draw sustenance therefrom by it 
own unaided efforts. The mother, as Profes 
Geoffroy and Mr. Morgan have shown, 
therefore provided with the peculiar adapta’ 
of a muscle (analogous to the cremaster) to 
mammary gland, for the evident pose of 
injecting the milk from the nipple into the 
mouth of the adherent fetus. Now it can 
scarcely be supposed that the feetal efforts of 
suction should always be coincident wi 
maternal act of injection; and if at a 
this should not be the case, a fatal ace 
might happen from the milk being foreit 
injected into the larynx, unless that u 
were guarded by some i 
Professor Geoffroy first described the 1 
fication by which this purpose is effected; and — 
Mr. Hunter appears to have anticipated the ne- 
COL 
