198 CO MP A HAT I VE HLiTORY OF F(ETAL MEMBRANES. 



allantoic arteries in the cord wind in a spiral manner round the 

 allantoic vein. The yolk-sack in many cases atrophies completely 

 before the close of intra-uterine life, but in other cases it is only 

 removed with the other embryonic membranes at birth. The intra- 

 embryonic portion of the allantoic stalk gives rise to two structures, 

 viz. to (1) the urinary bladder formed by a dilatation of its proximal 

 extremity, and to (2) a cord known as the urachus connecting the 

 bladder with the wall of the body at the umbilicus. The urachus, 

 in cases where the cavity of the allantois persists till birth, remains as 

 an open passage connecting the intra- and extra-embryonic parts of 

 the allantois. In other cases it gradually closes, and becomes nearly 

 solid before birth, though a delicate but interrupted lumen would 

 appear to persist in it It eventually gives rise to the ligamentum 

 vesica? medium. 



At birth the foetal membranes, including the foetal portion of the 

 placenta, are shed ; but in many forms the interlocking of the foetal 

 villi with the uterine crypts is so close that the uterine mucous mem- 

 brane is carried away with the foetal part of the placenta. It thus 

 comes about that in some placentae the maternal and foetal parts 

 simply separate from each other at birth, and in others the two 

 remain intimately locked together, and both are shed together as 

 the after-birth. These two forms of placenta are distinguished as 

 non-deciduate and deciduate, but it has been shewn by Ercolani and 

 Turner that no sharp line can be drawn between the two types ; 

 moreover, a larger part of the uterine mucous membrane than that 

 forming the maternal part of the placenta is often shed in the 

 deciduate Mammalia, and in the non-deciduate Mammalia it is pro- 

 bable that the mucous membrane (not including vascular parts) of 

 the maternal placenta either peels or is absorbed. 



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'. 



MarsupiaUa. 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 



1 The following is Owen's account of the young after birth [Coinp. 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." 



