ITS STRUCTURE. 95 



cells, one of which belongs to the maternal portion of the placenta, is 

 placed between the membrane of the villus and that of the vascular 

 system of the mother, and is probably designed to separate from the blood 

 of the parent the materials destined for the blood 

 of the foetus, while the other belongs to the feetal 

 portion of the placenta, is situated between the 

 membrane of the villus and the loop of vessels 

 contained within, and probably serves for the ab- 

 sorption of the material secreted by the other set 

 of cells, and for its conveyance into the blood- 

 vessels of the foetus. In describing these several 

 membranes and layers of cells as composing each 

 placental villus, Professor Goodsir calls the lining membrane of the vascular 

 system of the mother the external membrane of the villus (fig. 17 a), the 

 layer of cells between this and the villus, the external cells of the villus 

 (6), the membrane within these, the internal membrane of the villus (e\ 

 and the cells between this and the loop of vessels, the internal cells of the 

 villus (/). Between the two sets of cells with their investing membrane 

 there exists a space (d,} into which it is probable that the materials 

 secreted by the external cells of the villus are poured, in order that they 

 may be absorbed by the internal cells, and thus conveyed into the foetal 

 vessels. As the decidual vessels enlarge and extend themselves, so as to 

 ensheath the placental tufts, their lining membrane forms numerous folds, 

 the venous reflections of Dr. Reid, which pass from tuft to tuft, and villus 

 to villus, connecting them together in the same manner as the intestines 

 are tied together and held down in various places by reflections of the 

 peritoneum. These folds or processes of the venous membrane appear in 

 the form of innumerable threads passing from the sides or apex of one 

 villus to that of another throughout the substance of the placenta. On 

 minute examination, Professor Goodsir found that these threads were 

 tubular, and that the membrane of which they were formed was distinctly 

 continuous in one direction with the lining membrane of the enlarged 

 decidual vessels, and in the other with the external membrane of the 

 placental villi, forming, therefore, in this way, the central containing 

 membrane of the bag of the placenta. The tubular portion of the threads 

 was filled with cells, which were continuous in one direction with the cells 

 of the placental decidua, in the other with the external cells of the villi. 

 (See fig. 18, p. 96.) 



* Fig. 17. Extremity of a placental villus. a. External membrane of the villus, or 

 lining membrane of the vascular system of the mother, b. External cells of the villus. 

 c c. Germinal spots or centres of the external cells, d. Space between the maternal and 

 foetal portions of the villus. e. Internal membrane of the villus, or external membrane of 

 the chorion. /. Internal cells of the villus, or cells of the chorion. g. Loop of umbilical 

 vessels. After Goodsir. 



