FCETAL MEMBRANES 



439 



(Fig. 9). Thus the embryo is supplied by diffusion with the 

 necessities for existence during its comparatively long intra-uterine 

 life. 



Various forms of placenta are met with amongst the Placentalia. 

 The most primitive type is apparently that in which the allantois 

 becomes attached around the whole serosa, so that the resulting 

 chorion, from which the comparatively simple villi arise, are equally 

 distributed over the whole surface 

 (Fig. 333). This form is known 

 as a diffuse placenta, and is met 

 with in Manis, the Suidae, Hippo- 

 potamus, Tylopoda, Tragulidae, 

 Perissodactyla, and Cetacea. 



The next stage is characterised 

 by the chorionic villi becoming 

 more richly branched so as to 

 present a greater superficial ex- 

 tent, and at the same time con- 

 centrated into definite and more 

 or less numerous patches or 

 cotyledons. Thus a polycotylcdcm- 

 ary placenta arises, such as is met 

 with in most Ruminants, some of 

 which, such as Cervus mexicanus 

 and the Giraffe, show an interesting 

 intermediate form of placenta be- 

 tween the diffuse and the cotyle- 

 donary. 



The chorionic villi in these two 

 types of placenta, even when 



branched, separate from the uterine mucous membrane at birth, 

 the latter not becoming torn away; they are therefore spoken of as 

 non-deciduate. 



A further complication is seen in the forms of placenta known 

 as zonary, the dome- or bell-shaped, and the discoidal, in which the 

 connection between foetal and maternal parts becomes much more 

 close, the villi giving rise to a complicated system of branches 

 within the uterine mucous membrane (Fig. 334). Thus the latter 

 becomes to a greater or less extent torn away at birth, forming a 

 decidua, the placenta being therefore spoken of as deciduate. In 

 these cases, the placental part of the chorion does not extend all 

 round the embryo. In the zonary placenta only the two opposite 

 poles of the chorion are more or less free from vascular villi, and 

 this girdle-like form occurs in the Carnivora, as well as in the 

 Elephant, Hyrax, Orycteropus, and Halicore. In Lemurs and 

 Sloths, the placenta is dome- or bell-shaped, while in Myrmeco- 

 phaga, Dasypodidie (Armadilloes), and Primates (Fig. 9) it forms 

 a discoidal mass on the dorsal side of the embryo. In Rodentia, 



FIG. 333. DIAGRAM OF THE POSTAL 

 MEMBRANES OF A PLACENTAL MAM- 

 MAL. (From Boas's Zoology. ) 



al t allantois ; am, amnion ; b, yolk- 

 sac (umbilical vesicle) ; the outer- 

 most line represents the serous 

 membrane. The outer wall of the 

 allantois has united with the serous 

 membrane to form the chorion from 

 which villi arise. 



