THE EARLY DEVELOPMENT OF THE MAMMAL 449 



in the intra-placental cavity filled with maternal blood (black 

 in Fig. 185). Beneath this are the greatly reduced layers of 

 the decidua basalis, traversed by the maternal blood vessels 

 communicating with the intra-placental cavity. In the deeper 



layer of the decidua basalis portions of the uterine glands 

 remain present. Finally the decidua basalis rests upon the 

 muscular layer of the uterine wall (oblique ruling). 



Peripherally the irregular chambers of the intra-placental 

 cavity are nearly or quite free from villi ; and form the marginal 



