THE F<ETAL MEMBRANES OF MAMMALS. 235 



corresponding thickenings of the uterine mucous membrane, the 

 placentae uterinse (C 1 ), though only in a loose manner, so that a little 

 pulling is sufficient to produce a separation, and to draw the chorionic 

 villi out of the depressions which serve for their reception, as one 

 draws the hand out of a glove. In fact, in the preparation which 

 serves as the basis of our figure 135a the cotyledons of offspring and 

 mother ((7 2 and (7 1 ) are separated from each other, since the uterus 

 ( U) has been opened by means of an incision and drawn back from 

 the chorion (CJi) for a little distance. 



Figure 135b shows a single cotyledon of figure 135a somewhat 

 larger than the natural size. The wall of the uterus (u) is drawn 

 back a little from the chorion (CJi). As a result of this, the maternal 

 (C 1 ) and foetal parts ((7 2 ) of the cotyledon are partially separated 

 from each other. On the placenta uterina (C l ) one perceives many 

 small pits, on the placenta fretalis (C 2 ) the closely packed dendritically 

 branching chorionic villi, which have been withdrawn from the 

 pits. 



As the diagrammatic section figure 136 teaches, the foetal and 

 maternal tissues abut immediately on each other. The villi are 

 covered with flattened cells, and the depressions of the mucous 

 membrane are lined with cylindrical cells ; the latter develop within 

 them granules of fat and albumen ; they disintegrate in part, and 

 thereby contribute to the formation of a milky fluid, the so-called 

 uterine milk, which can be pressed out of the placenta uterina and 

 serves for the nutrition of the foetus. It is to be noticed also that 

 in the Ruminants the uterine glands have openings on the mucous 

 membrane only between the cotyledons. 



In all other Mammals that are provided with a placenta the 

 intergrowth of the foetal and maternal tissue is still more intimate. 

 At the same time there is formed in this way such a close union, 

 that a separation of the chorion without injury to the mucous membrane 

 of the uterus is now no longer possible. At birth therefore a more or 

 less considerable superficial layer of the mucous membrane of the uterus 

 is cast off" with the foetal placenta. The part that is cast off is called 

 the caducous membrane, or the decidua. 



In accordance with HUXLEY'S proposal, all Mammals in which, in 

 consequence of the special growth of the placenta, such a membrane 

 is formed are now grouped together as Mammalia deciduata, or 

 briefly Deciduata, in contradistinction to the remaining Mammals 

 the Indeciduata, the formation of whose placentae has just been 

 discussed. 



