MAMMALIA. 239 



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 vesicae medium. 



At birth the foetal membranes, including the fcetal portion of 

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

 the fcetal villi with the uterine crypts is so close that the uterine 

 mucous membrane is carried away with the fcetal part of the 

 placenta. It thus comes about that in some placentae the 

 maternal and fcetal 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 probable 

 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 fcetal 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 1 . 



1 The following is Owen's account of the young after birth (Comp. Anat. of 

 Vertebrates, Vol. in. p. 717) : " On the eighth of December Dr Bennet discovered in 

 "the subterranean nest of Ornithorhyncus three living young, naked, not quite two 



