12 COMPARATIVE ANATOMY. 



occupy a zonary area, or even so as to completely surround the foetus, when 

 it is spoken of as diffused. By the concentration of the villi of the diffused 

 placenta into definite patches, or cotyledons, a polycotyledonary form is 

 produced. 



Rodents, Insectivores, and Bats possess a discoidal deciduate placenta, 

 and that of Sloths, Armadilloes, and Myrmecophaga approaches the same type. 

 In Carnivores, Elephants, Hyrax,and Orycteropus, it is deciduate and zonary. 

 In all other Mammals the placenta is non-deciduate, the maternal and foetal 

 parts simply separating from one another at birth. Amongst these the poly- 

 cotyledonary form is found in the Ruminants proper, and the diffused form 

 in Suidae, Hippopotamus, Perissodactyla, Tylopoda, Tragulidsc, Manis, 

 Lemuridse, Sirenia, and Cetacea. The mode of development of the meta- 

 discoidal placenta of Primates shows that it has been derived from a diffused 

 placenta, the villi becoming restricted in the course of development to a 

 disk-shaped area, and their complexity increasing at the same time (Balfour). 



In the course of development the embryo becomes more and more folded 

 off from the yolk-sac (umbilical vesicle) (Fig. 9, Do], the stalk of which latter, 

 and that of the allantois, enveloped by the base of the amnion, together form 

 the umbilical cord. At birth, the foetal membranes are shed, the intra- 

 abdominal portion of the allantois persisting as the urinary bladder and the 

 urachus (cp. the chapter on the vascular system). 



Amongst Elasmobranchs, Mustelus laevis and Carcharias possess a kind of 

 placenta formed by the yolk-sac, which becomes raised into folds fitting into 

 the vascular walls of the oviduct. Indications of such an umbilical placenta 

 are also seen in the early stages of Insectivora, Cheiroptera, and Rodentia. 



Further investigations on the umbilical sac and allantois in Marsupials and 

 Moiiotremes are necessary. Indications of an umbilical placenta have been 

 observed in the former group. 



