DEVELOPMENT OF THE FETAL MEMBRANES AND THE PLACENTA 63 



strata of ectodermal cells derived from the trophoblast: the deeper stratum, 

 next the mesodermic tissue, represents the cytotrophoblast or layer of Langhans; 

 the superficial, in contact with the maternal blood, the syncytiotrophoblast (Figs. 

 36 and 37). After the fifth month the two strata of cells are replaced by a 

 single layer of somewhat flattened cells. 



Maternal Portion. The maternal portion of the placenta is formed by the 

 decidua placentalis containing the intervillous space. As already explained, this 

 space is produced by the enlargement and intercommunication of the spaces in 

 the trophoblastic network. The changes involve the disappearance of the greater 

 portion of the stratum compactum, but the deeper part of this layer persists and 

 is condensed to form what is known as the basal plate. Between this plate and 

 the uterine muscular fibres are the stratum spongiosum and the boundary layer ; 



Wall of uterus 



Umbilical cord 



Cervix uteri 





FIG. 38. Fetus in utero, between fifth and sixth months. 



through these and the basal plate the uterine arteries and veins pass to and from 

 the intervillous space. The endothelial lining of the uterine vessels ceases at the 

 point where they terminate in the intervillous space which is lined by the syncytio- 

 trophoblast. Portions of the stratum compactum persist and are condensed to 

 form a series of septa, which extend from the basal plate through the thickness 

 of the placenta and subdivide it into the lobules or cotyledons seen on the uterine 

 surface of the detached placenta. 



The fetal and maternal blood currents traverse the placenta, the former passing 

 through the bloodvessels of the placental villi and the latter through the inter- 

 villous space (Fig. 39). The two currents do not intermingle, being separated from 

 each other by the delicate walls of the villi. Nevertheless, the fetal blood is able 

 o absorb, through the walls of the villi, oxygen and nutritive materials from the 



