Development of the .Marsupiulia.—Before proceeding to detail the present received doc trine of the generation and development of the AIarsupialia, it may not be unprofitable to take a rapid glance at the different opinions that have prevailed at different periods respecting this interesting and difficult part of their economy.
The minute size of the young of the Ameri can Opossum when first received in the mar supium, their pendulous attachment to the nipples, and perhaps the mode in which the nipples themselves are developed, gave rise, among the earlier observers, to a notion that the young were originally formed by and from those parts.
And this belief was not only current then, as now, among the unscientific settlers in the colo nies where the marsupial animals are common, but was entertained likewise by the best in formed Naturalists of those times. Thus the learned Marcgrave, in his account of the Opos sum, says, when speaking of the marsupial pouch, " Here bursa ipse uterus at Animulis, num chum non habet, uti ex sectione illius coin peri; in hac semen concipitur et catuli forman tur." § And Piso repeats the assertion more strongly. " Er reiteratis horum unintaliuni sectionihs alium non invenimus uterus 'weer hanc bursam, in qua semen concipitur et catuli jitrmantur. Qum deinde quinos vet saws simul eircumfert, mobiles, perfectos, sed depiles, adro guc pertinacitir uberibus allixos, ut u papaw) man vix avellantur, anlequam permittente mutre ad pastum ipsi egrediantur." The assertion that the young grow from the nipple was again repeated in regard to the Philander Opossum ( Didelphys Philander) by Valentin in his History of Amboyna, and has even been revived at a comparatively recent period.t Some glimpses of the truth were ob tained, however, before the time of the authors who have been last quoted. Hernandez, for example, speaks of the generation of the Opossum almost in the same words in which Cuviert sums up the then existing know ledge of the subject in the second edition of his 'agile Animal.' " Quaternos, quinosve pant catulos, quos ulero conceptos, editosque in alvi capacitate quildam, duet adhuc par vuli sant, claudit ac servat."§ And Maffeius more particularly describes the attachment of the young to the nipple. " Illud autem mirum in Cerigonibus" (Opossums) "ex ejusalvo due' dependent veluti manticre, in its catulos cir cumfert, et quidem adeo pertinaciter suoquem que uberi affixos, ut a perpetuo suctu non avellantur, antequam ad pastum ipsi per se progredi valeant." II
Nevertheless, as the uterine gestation is here simply alluded to without any detailed obser vations in proof of it, the assertion was compara tively of little value in a scientific point of view; and the gemmiparous theory, supported by Marcgrave and Piso, seems to have been pre valent at the time when Dr. Tyson first turned his attention to this subject.
The discovery of the true uterus, recorded by that learned and accurate anatomist in the 20th volume of the Philosophical Transactions, p. 105, was the first step towards a correct theory of the generation of the marsupial ani mals. It necessarily caused him to reject the gemmiparous theory, but, as often happens in such cases, Tyson was led into the opposite or sceptical extreme; and he was also induced to doubt the really accurate statements of Her nandez and Arafircius respecting the function of the marsupial pouch; " for," says Tyson, " here I find they place the mamma; or teats, and they tell very odd stories about it," &c.
The female Opossum which Tyson dissected appears to have been a young one, and there fore, for a reason which has lately been clearly explained by Mr. Morgan, he was unable to detect the nipples within the pouch, and although he confesses that he was equally unable to find them upon the outer skin, be rejected the state ments respecting the premature birth of the young and their pendulous attachment to the nipple, and, believing the generation of the Opossum not to deviate from that of ordinary quadrupeds, he limited the function of the marsupium to that of affording a temporary shelter to the young in time of danger.
The assertions of Hernandez and Maffeius were soon, however, corroborated by other ob servers ; and Daubentons repeated and con firmed the dissections of Tyson, so far as re garded the existence and general form of the uterus; but no satisfactory explanation was offered as to the nature or precise period of the uterine development or of the passage of the young to the marsupium.