THE STRUCTURE OF THE HUMAN PLACENTA. 51 



rently devoid of any structure. This membrane may be described 

 as bounding the whole tuft, passing from the trunk to the branches, 

 and from these to the villi, the free extremities of which it closely 

 covers. Its free surface is smooth and glistening, its attached 

 surface is somewhat rough.* 



3. Of the External Cells of the Villi. 



Immediately under the membrane just described is a layer of 

 cells.f They are flattened spheroids, slightly quadrilateral in out- 

 line, from the manner in which they are packed together. When 

 a tuft is viewed in profile, under compression, its edges exhibit 

 the appearance of a double line, wilich leads the observer to sup- 

 pose that its bounding membrane is double, with the cells just 

 described situated between the two laminse. In the space be- 

 tween the two lines, the nuclei of the cells may be seen in the 

 form of dark oval spots, and the septa formed by the walls of con- 

 tiguous cells are also visible. 



At variable distances the space between the two lines widens 

 out into a triangular form, the base towards the external mem- 

 brane, the apex towards the centre of the villus. This wider space 

 is produced by a larger group of cells, which appear to be passing 

 off from a spot in the centre of the mass. The groups of cells I 

 am now describing are germinal spots. They are the centres 

 from which new cells are constantly passing off, to supply the 

 loss of those which have disappeared in the performance of their 

 important function. 



As in the case of the intestinal epithelium, I am inclined to be- 

 lieve that a fine membrane lines the internal aspect of the layer 

 of cells. I have not been able to isolate it ; but the very sharp 

 outline in a profile view of a villus confirms me in my belief of the 

 existence of such a membrane. 



* Professor Reid, " On the Anatomical Relations of the Blood- Vessels of the Mother 1o 

 those of the Foetus in the Hitman Species" Ed. Med. Surg. Journal, 1841, page 7. 



t Mr. Dalrymple, " On the. Structure of (he Placenta. 1 " Med. Chir. Trans. London, Vol. 

 xxv., pages 23, 24. 



