The next really important advance towards the solution of this problem was made by John Hunter, who in the dissection of some marsupial fetuses of the Kangaroo detected evidences of a deviation from the ordinary mode of mammiferous development, in the ab sence of the usual traces of a placental organi zation ; there being in these fcetuses no per ceptible remains either of an urachus, of umbi lical arteries, or of an umbilical vein. The beautiful series of preparations-1 exhibiting these and other interesting facts in the structure of the mammary kilts of the Kangaroo are pre served in the Museum of the Royal College of Surgeons, and afforded the principal materials for the paper on Marsupial Generation, pub lished by Mr. (afterwards Sir Everard) Ilome, in the 85th vol. of the Philosophical-Transac tions (1795). I have already shewn that one of the chief grounds of the theory of marsupial generation there proposed is untenable, the sup posed remains of the fcetus,§ described as being situated in the corpus uteri,(vaginal cul-de-sac,) being nothing more than a portion of the inspis sated secretion commonly present both in this sac and the lateral canals. The temporary ori fice by which the foetus is stated to pass imme diately from the so called corpus uteri into the vagina (nro-genital passage) does not exist.
In the subsequent theory of marsupial gene ration propounded by Sir Everard Home,* the cornua uteri' of Tyson are regarded as por tions of the Fallopian tubes. These he believes to furnish the yelk of the ovum, while the lateral canals, uteri reduplicati ' of Tyson, secrete the albumen ; the ovum is supposed to be impreg nated and incubated in the uterus, (middle cul de-sac formed by the communication of the two vaginal canals,) out of which the young one is stated to pass into the vagina (uro-genital pas sage) by a particular opening, which prior to gestation does not exist.
The only observations published by John Hunter himself relative to marsupial genera tion are contained in the Zoological Appendix to White's Voyage to New South Wales,' where, in the introduction to his descriptions of the quadrupeds of that country, Mr. hunter alludes to the American Opossum, and ob serves, " there is something in the mode of propagation in this animal that deviates from all others ; and though known in some degree to be extraordinary, yet it has never been at tempted, where opportunity offered, to com plete the investigation. I have often endea voured to breed them in England ; I have bought a great many, and my friends have assisted me by bringing them or sending them alive, yet never could get them to breed; and although possessed of a great many facts re specting them, I do not believe my information is sufficient to complete the system of propaga tion in this class." At this period, when it was admitted on all hands that some remarkable peculiarities were connected with the marsupial generation, and yet their precise nature and signification re mained unelucidated by any direct and accu rate observation or experiment, it is not sur prising that the subject should have given rise to many curious hypotheses and speculations ; those of Sir Everard Home have already been noticed. I shall next briefly allude to the writings of M. Geoffroy St. Hilaire.
The fruitful and discriminating labours of this talented Naturalist in advancing the zoolo gical history of the Marsupialia cannot be too highly esteemed, but his attempts to elucidate their generative economy have been less suc cessful.
Placing an undue reliance on the relation of Count Aboville,t he first revived the gem miparous doctrine4 meeting the objection afforded by Tyson's discovery of an uterus, by the remark that the fcetus of the marsupial animal has never been found there ; but that the teats are developed in the ratio of the size and according to the number of the young : that mules equally possess a generative appa ratus, which is stricken with sterility : that some plants with a perfect system of procrea tive organs, nevertheless propagate by gemma lion, &c. This theory, however, was aban
doned, or at least considerably modified after the publication of Dr. Barton's letters relative to the breeding of the Opossum. The product of marsupial generation is then stated by M. Geoffroy to quit the uterus in the condition of a gelatinous ovulum, comparable to a Medusa,and to become organically connected with the nipple by continuity of vessels. He supposed, there fore, that a flow of blood followed the detach ment of the mammary foetus from the nipple," and even speculates upon the signification of the thyroid gland, on the strength of this hypo thetical confluence of the maternal and foetal vascular systems in the marsupial tribe.t In another essay Al. Geoffroy abandons this opinion, it having been proved by repeated observation that the adhesion of the foetus to the nipple is by simple contact merely;t and filially, he falls into the opposite extreme, and from some appearances of an machus which were pointed out to him in a very small foetus of an American Opossum, he describes them as vestiges of aplacental organization.
Mr. Morgan,§ in his elaborate and excellent Memoirs on the Structure and Development of the Mammary Organs of the Kangaroo, bears testimony to the simply mechanical mode of attachment between the mammary foetus and the nipple in the Kangaroo, and was the first to show that the young animal, in its blind and naked condition, prior to the act of volun tary detachment,—the birth ' it la manihre des Dlarsiipiaux; as it is called by M. Geoffroy, would bear a separation from the nipple for two hours, and voluntarily regain its hold. The mammary fetus of the Kangaroo on which Mr. Alorgan experimented was nearly the size of a fully grown Norway rat. Mr. Collie, surgeon R.N. in a letter addressed from New South Wales, and published in the Zoological Journal, (No. xvirtr. p. 230,) obtained the same result in detaching from the nipple of a smaller species of Kangaroo ( Alacropas Brunie) a mammary fetus, not two inches in length : he says, " I gently pressed with the tip of my finger the head of the little one away from the teat of which it had hold, and continuing press ing a little more strongly for the space of a minute altogether, when the teat which had been stretched to more than an inch came out of the young one's mouth, and showed a small circular enlargement at its tip, well adapting it for being retained by the mouth of the sucker." —" An hour afterwards the young was ob served still unattached, but in about two hours it had hold of the teat, and was actively em ployed sucking." Dr. Barton's very interesting observations on the American Opossum are as follows :—" The female Didelphis Woapink sometimes produces sixteen young ones at a birth. I have actually seen this number attached to the teats, but never a greater number. When they are first excluded from the uterus, they are not only very small but very obscurely shaped. The place of the future eyes is merely marked by two pale bluish specks; we see no ears ; in short the animal is a mere mis-shaped embryon. Its mouth, which is afterwards to become very large, is at first a minute hole, nearly of a trian gular form, and just of a sufficient size to receive the teat, to which the little creature adheres so firmly, that it is scarcely matter of surprise that Beverly* and other writers have asserted that the young grow fast to the teats.