REPRODUCTIVE ORGANS 81 



Sirenia present the peculiarity of having a zonary placenta, which is 

 either entirely or in great part non-deciduate, and is, therefore, 

 transitional between the diffused and the true zonary type. 



In the true Ruminants or Pecora, among the Artiodactyle 

 Ungulates, the villi are aggregated in masses called cotyledons, 

 with bare spaces between. Such a placentation is called "poly- 

 cotyledonary." In another modification the villi are collected in a 

 more or less broad band encircling the chorion, leaving a very large 

 portion of the two poles bare, constituting the "zonary placenta," 

 characteristic of the Carnivora, and also occurring in the Elephant, 

 Hyrax, and Orycteropus. The fact of the form of the placenta of 

 these three last-named animals agreeing together, and with that of 

 the Carnivora, does not, however, necessitate the ascription of 

 zoological affinities, as the same ultimate form may have been 

 attained by different processes of development. 



In another form one pole only of the chorion is non-vascular, 

 the placenta assuming a dome or bell shape, as in the Lemurs and 

 the Sloths. The transition from this, by the gradual restriction of 

 the vascular area, is easy to the oval or discoidal form of placenta, 

 of the Anteaters, Armadillos, and higher Primates. The discoidal 

 placenta of the Rodents, Insectivores, and Chiroptera, though show- 

 ing so much superficial resemblance to that of the last-named order 

 as to have led to the inclusion of all these forms in one primary 

 group, is now known to be developed in another manner, not by the 

 concentration of villi from a diffused to a limited area, but by 

 retaining the area to which it was originally restricted in con- 

 sequence of the large surface of the chorion occupied, as before 

 mentioned, by the umbilical vesicle. To compensate for the small- 

 ness of area, the complex or deciduate structure has been developed. 

 Among some Rodents there is evidence to show that the discoidal 

 placenta has been derived from a zonary one, of which distinct 

 vestiges have been detected in the Mouse. We may conclude 

 that, although the characters and arrangement of the foetal structures 

 may not have that extreme importance which has been attributed 

 to them by some zoologists, they will form, especially when more 

 completely understood, valuable aids in the study of the natural 

 affinities and evolution of the Mammalia. 1 



1 For a full exposition of the present state of knowledge on this subject, see 

 the various memoirs of Sir William Turner, also F. M. Balfour's Treatise on 

 Comparative Embryology, vol. ii. (1881), and J. A. Ryder in American Naturalist, 

 vol. xxi. p. 780 (1887). 



