INDECIDUA AND DECIDUA. l6l 



minute and scattered tufts, or none at all ; while the latter 

 is thickly overgrown with highly developed and large tufts. 

 In the Deciduata the tufted chorion alone forms the 

 placenta. 



Yet more characteristic of the Deciduata is the very 

 peculiar and intimate connection which is developed in 

 these between the tufted chorion and the contiguous 

 portion of the uterine mucous membrane, and which must 

 be regarded as a true coalescence. The vascular tufts of 

 the chorion push their branches into the sanguineous tissue 

 of this mucous membrane in such a way, and the two sets 

 of vessels are in such close contact and are so interlaced, 

 that the embryonic placenta is no longer distinguishable 

 from the maternal placenta ; the two form one whole — a 

 compact and apparently simple placenta. Ow.'ng to this 

 intimate coalescence, a portion of the uterine mucous mem- 

 brane of the mother comes away, at birth, with the firmly 

 adherent egg-membrane. The portion of the mother's body 

 which is thus removed in parturition is called, on account 

 of its separable nature, the deciduous membrane (decidua). 

 All Placental Animals which possess this deciduous mem- 

 brane are classed together as Deciduata. The removal of 

 this membrane at parturition, of course, causes a greater or 

 less loss of blood by the mother, which does not occur in 

 the Indecidua. In the Deciduata, moreover, the lost portion 

 of the uterine mucous membrane must be replaced, after 

 parturition, by a renewal of the tissue. 



The structure of the placenta and deciduous membrane 

 is, however, by no means identical throughout the compre- 

 hensive group of Deciduata. On the contrary, there are 

 many important differences in this respect, which are in 



