[91] According to Bischoff the subzonal membrane atrophies, leaving the allantoic mesoblast to constitute the whole chorion.
[92] The observations on this head were made by Sharpey, and are quoted by Huxley (No. [202]) and with additional observations by Turner in his Memoir on the placentation of the Sloths. Anderson (No. [191]) has also recently confirmed Sharpey’s account of the diffused character of the placenta of Manis.
[93] Entwicklungsgeschichte des Menschen, etc., 2nd ed., p. 362. Leipzig, 1876.
[94] Schäfer’s and Hensen’s statements are in more or less direct contradiction as to the structure of the ovum after the formation of the embryo; and it is not possible to decide between the two views about the ovum till these points of difference have been cleared up.
[95] According to Hensen the hypoblast grows round the inside of the wall of the cylinder from the body which he regards as the ovum. The original wall of the cylinder persists as a very thin layer separated from the hypoblast by a membrane.
[96] Hensen states that the hypoblast never grows into the allantois; while Bischoff, though not very precise on the point, implies that it does; he states however that it soon disappears.
[97] Allen Thomson informs me that he is very confident that such a form of attachment between the hind end of the embryo and the wall of the vesicle, as that described and figured by His in this embryo, did not exist in any of the younger embryos examined by him.
[98] For a fuller explanation of His’ views I must refer the reader to his Memoir (No. [232]), pp. 170, 171, and to the diagrams contained in it.