514 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



body of the female, but also pass through the early 

 stages of development within it, the ducts become 

 much more elaborate than in. the oviparous forms ; 

 Vertebrates in which this obtains are, somewhat im- 

 properly, spoken of as viviparous. The most pro- 

 minent phenomenon is the enlargement of the distal 

 portion of the duct into a cavity of varying width, the 

 so-called uterus. In most of the viviparous fishes, 

 in the few viviparous Lacertilia (Zootoca, Anguis 

 fragilis), and in the viviparous Ophidia, the ova appear 

 to continue to develop within the uterine cavity with- 

 out any assistance or nourishment from the mother. 



In a few Elasmobranchs, however, and in all the 

 higher Mammalia, means are taken by which the 

 young are brought into real relation with the parent, 

 arid are directly nourished by its blood ; in the 

 former (e.g. Mustelus Isevis) the vascular walls 

 of the yolk sac become raised into ridges which 

 fit into corresponding depressions in the vascular 

 uterine membrane. Here we have the lowest and 

 simplest representation of a placenta. 



Of the aplacental Mammals little is definitely 

 known, and what information we have was long ago 

 furnished by Owen, who observed in Ornithorhynchus 

 that the ova lay free in the uterus, the lining 

 membrane of which was highly vascular ; in a foetal 

 kangaroo there were folds on the investing membrane 

 of the foetus, and corresponding depressions on the 

 walls of the uterus, but no organic connection was 

 observed between the parent and the young. 



In the placental Mammals outgrowths of the 

 investing membranes of the egg become more or less 

 closely united with vascular outgrowths of that 

 portion of the oviduct which has the function of a 

 uterus, and give rise to a characteristic organ. 



To make such structures intelligible it is necessary, 

 first of all, to give some account of the characters of 



