XIII PHYLUM CHORDATA 595 



the embryo are termed the fcdal placenta, those developed from the 

 wall of the uterus the maternal placenta. In some Mammals the 

 union between the two is not vei-y close, so that at birth no part 

 of the uterine mucous membrane is thrown off; such a placenta is 

 said to be non-deciduate {semi-placenta). In other Mammals the 

 union is closer, and at birth a part of the hypertrophied mucous 

 membrane is thrown off in the form of a decidua ; such a placenta 

 is termed deciditate (placenta vera). In the Mole and the 

 Bandicoot not only is there no decidua thrown off, but the 

 foetal placenta with the distal portion of the allantois does not 

 pass out after the foetus, but remains, and is broken up or absorbed 

 in the uterus. Such a condition has been termed contra- 

 deciduate. 



In one of the simplest forms of placenta — the discoidal — found 

 in the Rabbit and other Rodents (Fig. 1219), the yolk-sac extends 

 over the surface of the serous membrane and fuses with it, except 

 in a small area on the dorsal side of the embryo. In this small 

 area the allantois becomes applied to the chorion and coalesces 

 with it; and from the membrane so formed vascular villi grow 

 out, and are received into the uterine crypts. In most Mammals, 

 however, as already stated, the allantois becomes applied to the 

 chorion throughout its entire extent, and thus completely encloses 

 the embryo. Villi may be developed from all parts except the 

 poles : when this condition persists in the fully-formed placenta, 

 the term diffuse is applied. Sometimes the diffuse condition is 

 temporary, and the completed placenta has villi disposed in a 

 broad band or zone (zonary placenta). Sometimes the villi are 

 grouped together in patches or cotyledons (cotyledonary placenta). 

 In Man and the Apes the villi become secondarily restricted to 

 a disc-shaped area of the chorion situated on the ventral side 

 of the embryo (meta-discoidal placenta). (Cf. pp. 479-486.) 



In many Mammals the yolk-sac, through the medium of the 

 chorion, enters into a close relationship with the uterine wall, and 

 a connection, the so-called yolk-sac placenta, is established, through 

 which nourishment can be conveyed to the embryo ; but this 

 rarely persists after the true (allantoic) placenta has become 

 established. 



The stalk of the yolk-sac, with the corresponding narrowed part 

 of the allantois and the vessels which it contains, forms the 

 umbilical cord by which the foetus is connected at the navel or 

 umbilicus with the yolk-sac and placenta. This is enclosed in a 

 sheath formed by the ventral portion of the amnion. The part of 

 the allantois which remains within the cavity of the body develops 

 into the urinary bladder, together with a cord — the urachus — 

 connecting the bladder with the umbilicus. 



The developmental history of the Marsupials differs from that 



