Chap, xiii.] PLACENTA OF MAMMALS. 517 



therefore, to follow Balfour in applying the term 

 metadiscoidal to the disc-shaped placentae of such 

 forms as have the placenta formed by the allantois, 

 rather than by the yolk sac. From the diffused or 

 discoidal forms, or from both, there has been evolved 

 another form of placenta, such as is seen in the Car- 

 nivora and some other mammals ; in these the villi 

 are confined to a broad girdle-shaped region of the 

 chorion, and we have therefore an arrangement which 

 may well be spoken of as zonary. 



The oviducts of all vertebrates are typically 

 paired, and in many cases, as in the Amphibia, they 

 are completely separate from one another along the 

 whole of their course, as they are also in the Reptilia, 

 although in the Chelonia their terminal portion is in- 

 vested in a common sheath. 



In Teleostei and Elasmobranchs the two ducts 

 unite at their termination to open by a common orifice; 

 in the Teleostei they open to the exterior behind the 

 anus ; in Elasmobranchs, Dipnoi, and all the Sauro- 

 psida, they open into a pit or cloaca, which is common 

 to them, the renal, and the rectal orifices. 



In the Monotremata, in which there is a distinct, 

 and the Marsupialia, in which there is a shallow, 

 cloaca, the oviducts open separately ; in the latter, 

 but not in the former, the lower part of the oviducal 

 tube is modified to form a vagina, which, however 

 modified in various forms, is essentially double. In 

 the higher Mammals, in which the common urino- 

 genital orifice is placed in front of the anus, and sepa- 

 rated from it by a more or less distinct perinseiiui, 

 the terminal portions of the two oviducts are always, 

 at least externally, and in most cases also internally, 

 single and united. As we ascend the soale of the 

 Eufcheria we find this confluence becoming more and 

 more extensive. In the great ant-eater there is a 

 remnant of the two walls of the separate tubes in 



