62 THE STRUCTURE OF THE HUMAN PLACENTA, 



mother, and in the other with the external membrane of the tufts 

 of the placenta, and passing from one tuft, or set of tufts, on to 

 another, so as to form the central containing membrane of the bag 

 of the placenta. These threads, as well as their cavities, are some- 

 what funnel-shaped at each extremity. The funnel-shaped por- 

 tions of the cavities of threads, and, in some instances, the whole 

 length of the tube, were found to be full of cells, which were con- 

 tinuous in the one direction with the parietal decidua of the pla- 

 centa, and in the other with the external cells of the placental villi.* 



This observation led me at once to perceive the real signi- 

 fication of the external cells of the placental tufts. I saw that 

 this great system of cells was a portion of the decidua, all but cut 

 off from the principal mass by the enormous developement of the 

 decidual vascular network, but still connected with it by the minute 

 files of cells, which fill the cavities of the placental threads. 



This system of cells, the external cells of the villus, with the 

 external membrane, are portions of the decidua, and, unlike the 

 other elements of the placental tufts, belong to the organism of 

 the mother. These cells, with their membrane, I name the 

 central division of the placental decidua, to distinguish it from 

 the other portion formerly described, and which I have already 

 called the parietal division of the placental decidua. 



1. Mv observations have confirmed the statements of Professors 

 Weber and Sharpey as to the mode of formation of the decidua 

 vera ; but have led me to attach more importance to the inter- 

 follicular substance, and to the secreted or non-vascular portion 

 of the decidua. 



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

 and of a maternal portion intermixed. But the maternal 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 portion of 

 the wall of the vascular system of the mother, continuous 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 be- 



* These are the reflections of the veinous membrane of the mother, described by Dr. Reid. 



