FCETAL MEMBRANES 



339 



allantoic placenta arises, consisting of maternal and foetal parts 

 (Fig. 9). Thus the embryo is supplied 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. 271). This form is known 

 as a diffused placenta, and is met with in Manis, the Suidse, 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 extent, 

 and at the same time being concentrated into definite and 



Mu-ttf-?. C^iillarea 



Bccio/ua. 



Cfhsrianepithel 





y^ifffj-?, Bhitg-efass 



Fi(!. 272. — Diagram to illustrate the Relations of the Fcetal and 

 Maternal Vessels in the Human Placenta, showing Chorionic and 

 Maternal Vessels and Capillaries, Villi (Zotten), and Decidua. 

 (After Keibel.) 



more or less numerous patches or cotyledons. Thus a polycoty- 

 ledonary 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 between the diffuse and 

 the cotyledonary. 



The chorionic villi in these two types of placenta, even though 

 more or less branched, separate from the uterine mucous membrane 

 at birth, the latter not becoming torn away : these placentse are 

 therefore spoken of as non-decidtiate. 



A further complication is seen in the forms of placenta known as 

 the zonary, the dotne- or hell-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. 272). Thus the latter 



z 2 



