352 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 



and that 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 no sharp line can be drawn 

 between the two types. Moreover, a larger part of the 

 uterine mucous membrane than that actually entering 

 into 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 

 is either shed or 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. Nothing is known of the arrangement 

 of the foetal membranes in the former group of animals 

 (Monotremata). In the latter (Marsupialia) the yolk- 

 sac is large and vascular, and is, according to Owen, 

 attached to the subzonal membrane. The allantois on 

 the other hand is but small, and is not attached to the 

 subzonal membrane ; it possesses however a vascular 

 supply. 



Observations have hitherto been very limited with 

 regard to the foetal membranes of this group of animals, 

 but it appears highly probable that both the yolk-sac 

 and the allantois receive nutriment from the walls of 

 the uterus. 



All Mammalia other than the Monotremata and 

 Marsupialia have a true allantoic placenta. The pla- 



