472 ZOOLOGY FOR MEDICAL STUDENTS chap. 



Amongst Mammals the Prototheria have not yet reached complete 

 viviparity. The egg is comparatively large (3-4 mm. in Echidna) and 

 is laid at a time when the embryo is still at a relatively early stage of 

 development. In Echidna the mother transfers the egg to the pouch, 

 where it remains until hatching takes place. In Ornithorhynchus the 

 egg is deposited at the inner end of the burrow but nothing is known 

 yet as to whether there is any incubation. 



In the Metatheria matters have advanced a stage further. The egg 

 has now lost nearly all its yolk and is reduced nearly to the size of the 

 Eutherian egg. Development goes on for a considerable time within the 

 uterus and in a few cases simple placental arrangements are developed 

 from allantois and yolk-sac but the young when born are at a far less 

 advanced stage of development than in the Eutheria. They attach 

 themselves to the teats, situated as a rule within a pouch but sometimes 

 freely exposed, and hang on to them continuously for a prolonged 

 period while they go on with their development. In the small pouchless 

 Opossums they remain with the mother, holding on by curling their 

 tails round hers, long after there is any apparent necessity, and she 

 presents a remarkable spectacle, almost hidden under the burden of a 

 numerous family of young approaching herself in size. 



In the Eutheria the most perfect condition of viviparity is reached. 

 The placenta shows interesting differences in the different groups of 

 Eutheria. These differences have to do in the first place with the general 

 form and relations of the placenta. The allantois, instead of merely 

 taking a somewhat mushroom form and giving rise to a discoidal placenta 

 as in the Rabbit, may continue to spread all round its edge so that 

 eventually the entire blastocyst wall is lined by allantois, very much as 

 is the case in the Bird except that in the mammal it is commonly only 

 the mesodermal covering of the allantois that becomes extended in this 

 way, the primitive cavity with its endodermal lining being reduced or 

 entirely absent. When such extension of the allantois or of its meso- 

 dermal sheath has taken place the villi over the whole surface of the 

 blastocyst may become vascularized and converted into placenta, instead 

 of merely the localized patch seen in the Rabbit. We thus get what is 

 termed a diffuse placenta — such as occurs in the Pig or the Horse. 

 Further modifications arise by secondary localization of placenta 

 formation. Thus in the cotyledonary placenta found in Ruminants the 

 placental structure is restricted to numerous little cushions or buttons 

 scattered irregularly over the surface of the blastocyst — their location 

 predetermined by special patches of the uterine lining. In the zonary 

 placenta of the Cat, Dog, and other Carnivora, on the other hand, the 



