" It is not true that the young cannot be detached from the mother without the loss of blood ; I can assert the contrary from many ex periments made upon embryons weighing nine grains and upwards. I have fully satisfied myself as to all the various circumstances, both in the structure and in the exertions of the minute animal, which enable it, while yet a mere speck of living matter, to cling so firmly to the fountain of its support.
" — The wonderful little Didelphis is by no means the inanimate or the passive being some physiologists and naturalists have repre sented it.t " The toes of the fore-feet of the new born embryon opossum are furnished with sharp and hard nails or claws, but this is not the case with the hind-feet. The latter are for some weeks of little use to the animal; but by means of the former it is enabled to cling most firmly to the teat, and especially to the hair in the marsupium immediately around the teat.
" —An o possu m-embryon, or fcetus,which weighed sixty-seven grains, lived upwards of thirty hours after I had detached it from the teat. Another which weighed 116 lived thirty eight hours, at which time I killed it by putting it in spit its.
" Superfcetation (I should perhaps in strict propriety say uterine superfcetation) is wholly incompatible with the established laws of the economy of the Didelphis. But Nature, always provident, wastes no time. While, therefore, the first litter of young opossums are fast ap proaching to their adult state, the mother ac cepts the ardour of the male; she is impreg nated ; and after a gestation which is not, I think, remarkably short, if we consider the small size of the embryons when they are ex cluded from the uterus, the marsupium is des tined to perform the office of a second, I was going to say a more important, uterus ; just at the time when the first litter have attained such a size that they are no longer (one or two of them at the utmost) capable of taking refuge in her pouch." Besides the satisfactory evidence, thus afforded by different and independent observers, respect ing the condition of the mammary foetus and its true relations to the nipple, the discovery of the uterine fcetus was announced nearly at the same time by two naturalists in two different species of marsupial animals. Mr. Collie, whose experiment on the young mammary foetus of a Kangaroo has just been quoted, states in the same letter, " I have just now procured two gravid uteri, (of the Macro pus Brunii,) in which foetuses seem to be arrived at, or very near to, the termination of the period of gestation. One of them, which
is about the size of the smallest young already mentioned, which was about one-half larger than the body of the common wasp, as being in the abdominal sac, has protruded through an opening inadvertently made in the uterus, and is distinctly seen through its transparent membranes and the liquor arrinii."-t About the same time Dr. Rengger, a naturalist who was detained several years by the Dictator Francia in Paraguay, gave the following account of the generation of a species of Opossum ( Didelphis Azarre) in his work on the Mammalia of that province. " The foetuses are developed in the cornua uteri, and not in the lateral canals. Some days after impregnation they have the form of small round gelatinous corpuscles, which do not appear, even when examined with a lens, to have any communication with the mother, but a red line indicates the first commencement of development. Towards the end of gestation, when the fcetnses have attained the length of six lines, they are seen to be en veloped in a membrane and provided with an umbilical chord, which is united to the uterus by the medium of many filaments. The head, the four extremities, and tail are recognizable with the naked eye, but those foetuses which are nearest the Fallopian tubes are generally least advanced. "In gestation they make the circuit of the lateral canals, in which they are found to be deprived of their foetal envelopes, and to have no communication with the parent by means of the umbilical chord ; whilst one foetus was found in this situation, two others were still in the body of the uterus (vaginal cul-de-sac), from which the umbilical chords were not yet detached. At this period a slight enlargement of the cul-de-sac and lateral canals was the only change perceptible in them.% Thus, by the various observations derived from the different sources above indicated, the following propositions are satisfactorily esta blished, viz. that the young of the Marsu pialia are developed, primarily, as Tyson con jectured, in the true uteri or cornua uteri ; but that, contrary to Tyson's opinion, they are, as compared with other Mammalia, prematurely born • and that, nevertheless, the attachment of the immature young to the nipple is essen tially the same as in ordinary mammals, the young marsupial being nourished by the lac teal secretion, and its blood aerated by its own independent respiratory actions.