STRUCTURE OF HUMAN PLACENTA. 459 



Professors Weber and Sharpey as to the mode of formation of 

 the decidua vera ; but have led me to attach more importance 

 to the interfollicular substance, and to the secreted or non- 

 vascular portion of the decidua. 



2. The placenta, as has long been admitted, consists of a 

 festal and of a maternal portion intermixed. But the mater- 

 nal portion, instead of consisting of a part of the vascular 

 system of the mother only, includes the whole of the external 

 cells of the villi. 



3. The external membrane of the placental villi is a por- 

 tion of the wall of the vascular system of the mother, continu- 

 ous with the rest of that wall, through the medium of the 

 placental threads and lining membrane of the placental 

 cavity. 



4. The system of the external cells of the placental villi 

 belongs to the decidua, and is continuous with the parietal 

 division through the medium of the cavities of the placental 

 threads. This portion of decidua has been named the central 

 division of the placental decidua, and the threads decidual 

 bars. 



5. The function of the external cells of the placental villi 

 is to separate from the blood of the mother the matter 

 destined for the blood of the foetus. They are, therefore, 

 secreting cells, and are the remains of the secreting mucous 

 membrane of the uterus. 



6. Immediately within the external cells of the placental 

 villi there is a membrane which I have named the internal 

 membrane of the villi. This membrane belongs to the system 

 of the foetus, and is the external or bounding membrane of 

 the villi of the chorion. 



7. Inclosed within the internal membrane of the placental 

 villi is a system of cells, which belong to the system of the 

 foetus, and are the cells of the villi of the chorion. These are 

 the internal cells of the placental villus. 



