562 



ZOOLOGY 



SECT. 



but remains, and is broken up or absorbed in the uterus. Such 

 a condition has been termed contra-deciduate. 



In the simplest form of placenta the discoidal found in the 

 Rabbit and other Rodents (Fig. 1150), the yolk-sac extends over the 

 surface of the serous membrane and becomes fused with it, except 

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

 area the allantois becomes applied to the serous membrane and 

 coalesces with it to form the chorion, from which vascular villi 

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

 Mammals, however, as already stated, the allantois becomes 

 applied to the serous membrane throughout its entire extent, 

 and the chorion thus completely encloses the embryo. Villi may 

 be developed from all parts of the chorion, except at 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 restricted to a disc-shaped 

 area of the chorion situated on the ventral side of the embryo 

 (meta-discoidal placenta). 



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

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

 ilinbiliccd cord by which the foetus is connected at the 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 um- 

 bilicus. 



The developmental history of the Mar- 

 supials differs from that of the Eutheria 

 in the transitory character or entire 

 absence of a placental connection between 

 the foetus and the uterine mucous mem- 

 brane. The intra-uterine development of 

 the foetus is abbreviated, and birth takes 

 place when the young animal is still re- 

 latively very small and has many of the 

 parts incompletely formed. In this help- 

 less condition the young Marsupial is 

 placed by the mother in the marsupium, 

 where it remains for a time as a mam- 

 mary fcetus (Fig. 1153), hanging passively 

 to the teat, to which the mouth becomes firmly adherent. The 

 milk is expressed from the mammary gland by the contraction 

 of a muscle, the cremaster, and passes down the gullet of the foetus, 



Fin. 1153. Mammary fcetus of 

 Kangaroo attached to the 

 teat. (Natural size.) 



