The Biology of Twins (Mammals) (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1917 Excerpt: ...Dasypus one can call a stock. The phenomenon is clearly that of fission." In support of the fission hypothesis, he cites evidence derived from the embryological study of other mammals, notably the sheep and the ferret. "In the sheep Ovis I found some years ago a blastocyst at the stage just before the formation of the embryonal areas with two distinct ectodermic masses lying within the trophoblast." His outline figures show this interesting sheep blastocyst (Figs. 35 and 36) to be covered by a complete envelope of trophoblast and lined internally with a complete layer of endoderm. Such a condition could not have resulted from budding. In the ferret (Putorius) certain interesting conditions (Figs. 37 and 38) were found that seemed to show evidences of a separation of blastomeres, but in no case were twin embryos produced. These cases are cited to prove "that fission of the embryonic rudiments of eutherian mammals may be effected fairly easily, but the occurrence is the exception, not the rule." A 1 Meaning Dasypus hybridus. Figs. 35 and 36.--Two views of sheep ovum with twin embryo (after Assheton). It is unlikely that such double embryos develop very far. constructive theory of polyembryony in Tatusia (Dasypus) is then offered, which is based upon the unique combination of three conditions: (1) the development of the blastocyst within the central lumen of the uterus which has allowed of a considerable expansion of the ectodermic plate, owing to the rolling up of the blastocyst cavity as in Lupus now; (2) 'inversion of layers' by which the ectoderm plate becomes invaginated into the large cavity of the blastocyst subsequent to its expansion; (3) a late formation of a thickened mass of trophoblast over the entire expanded plate puttin...