The vitellicle or umbilical sac presented the same large proportionate size and vascular structure as in the first described fmtus. The chorion which enveloped this fmtus 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 fmtus, 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 fmtus 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 andperfectly 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.
" I 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 I 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 foetus quits the pouch: it has, prior to this period, quitted the nipple, and occasionally protrudes its head and changes its position in the pouch.
The anatomical condition and progressive development of the mammary fcetus of the Marsupialia offer a subject of highly interest ing research, especially if compared with the same circumstances in the uterine fcetus of an equal sized and analogous placental species.
Much still remains to be done in this chapter of the history of Marsupial generation ; at present I have to offer the following obser vations.
By comparing the new-born Kangaroo with a similarly sized fcetus 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 fcetus, 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 fcetus, 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 the abdominal parietes, form ing an anterior ligament of the bladder. In a mammary fcetus of the Kangaroo about a month older than the above, there was at the superior part of this duplicature a small projecting point from 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 foetuses of the illersupialia. In the Sloth, the Mania, 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.