_Comparative history of the Mammalian foetal membranes._ Two groups of Mammalia--the Monotremata and the Marsupialia--are believed not to be provided with a true placenta. The nature of the foetal membranes in the Monotremata is not known. Ova, presumably in an early stage of development, have been found free in the uterus of Ornithorhyncus by Owen. The lining membrane of the uterus was thickened and highly vascular. The females in which these were found were killed early in October[85]. [85] The following is Owen's account of the young after birth (_Comp. Anat. of Vertebrates_, Vol. III. p. 717): "On the eighth of December Dr Bennet discovered in the subterranean nest of Ornithorhyncus three living young, naked, not quite two inches in length." On the 12th of August, 1864, "a female Echidna hystrix was captured ... having a young one with its head buried in a mammary or marsupial fossa. This young one was naked, of a bright red colour, and one inch two lines in length." Marsupialia. Our knowledge of the foetal membranes of the Marsupialia is almost entirely due to Owen. In Macropus major he found that birth took place thirty-eight days after impregnation. A foetus at the twentieth day of gestation measured eight lines from the mouth to the root of the tail. The foetus was enveloped in a large subzonal membrane, with folds fitting into uterine furrows, but _not adhering to the uterus, and without villi_. The embryo was enveloped in an amnion reflected over the stalk of the yolk-sack, which was attached by a filamentary pedicle to near the end of the ileum. The yolk-sack was large and vascular, and was connected with the foetal vascular system by a vitelline artery and two veins. The yolk-sack was partially adherent, especially at one part, to the subzonal membrane. No allantois was observed. In a somewhat older foetus of ten lines in length there was a small allantois supplied by two allantoic arteries and one vein. The allantois was quite free and not attached to the subzonal membrane. The yolk-sack was more closely attached to the subzonal membrane than in the younger embryo[86]. [86] Owen quotes in the _Anatomy of Vertebrates_, Vol. III. p. 721, a description from Rengger of the development of Didelphis azaræ, which would seem to imply that a vascular adhesion arises between the uterine walls and the subzonal membrane, but the description is too vague to be of any value in determining the nature of the foetal membranes. All Mammalia, other than the Monotremata and Marsupialia, have a true allantoic placenta. The placenta presents a great variety of forms, and it will perhaps be most convenient first to treat these varieties in succession, and then to give a general exposition of their mutual affinities[87].