362 



REPRODUCTION 



eroded spaces, where it comes to lie in contact with the 

 syncytium. 



Internal to the basal layer is a layer of mesoblast. 

 These two layers send out processes which ramify in the 

 syncytium to form the chorionic villi. In the mesoblast 

 are laid down blood-vessels which become connected with 

 the foetal circulation by the umbihcal vessels. 



The above changes involve both the decidua serotina 



uterine vessel- 



SiWmuc. layeK^ 

 of uterus 



decidua — 

 syncytium 



syncytium. 



basal layer, 



mesoblast 

 of chorion 



blood space 



Fig. 70. — Diagrammatic section of the decidua serotina and chorion 

 to show how the placental blood spaces are formed (Keith). 



and the decidua reflexa. In the third month the hyper- 

 trophy of the chorionic vilh in the former and their atrophy 

 in the latter lead to the formation of the placenta. 



In the fifth month the growth of the villi ceases and in 

 place of the basal layer and syncytium there appears a single 

 layer of flattened cells. On one side of this membrane is the 

 foetal blood circulating in the chorionic villi, on the other 

 side is the maternal blood circulating in the lacunae eroded 

 out of the decidua by the syncytium. Across the mem- 

 brane the food material and oxygen are transferred from 



