[81] Schäfer describes the blastodermic vesicle of the cat as being throughout in a bilaminar condition before the formation of a definite primitive streak or of the mesoblast.
[82] This figure was drawn for me by my pupil, Mr Weldon.
[83] The hypoblastic element in the allantois is sometimes very much reduced, so that the allantois may be mainly formed of a vascular layer of mesoblast.
[84] These crypts have no connection with the openings of glands in the walls of the uterus. They are believed by Ercolani to be formed to a large extent by a regeneration of the lining tissue of the uterine walls.
[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.”
[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 fœtal membranes.
[87] Numerous contributions to our knowledge of the various types of placenta have been made during the last few years, amongst which those of Turner and Ercolani may be singled out, both from the variety of forms with which they deal, and the important light they have thrown on the structure of the placenta.
[88] Vide Ercolani, No. [197], and Harting, No. [201], and also Von Baer, Entwicklungsgeschichte table on p. 225, part I., where the importance of the limited area of attachment of the allantois as compared with the yolk-sack is distinctly recognised.
[89] This is denied by Nasse; vide Kölliker, No. [183], p. 361.