EXPLANATION OF THE PLATES. xix 



Fig. 21. A portion of ike external membrane, with external cells of 



the villus. 



a. Cells seen through the membrane. 

 6. Cells seen from within the villus. 

 c. Cells seen in profile along the edge of the villus. 

 Fig. 22. The extremity of a villus treated with acetic acid. All the 

 parts are distinctly visible, and the germinal centres of the 

 internal cells are seen surrounding the umbilical vessel. 

 Fig. 23. A villus with a terminal decidual bar, along the cavity of 

 which the external cells are seen to be continued, so as to 

 pass forwards in the direction of the parietal decidua. 



PLATE VI. page 445. 



Fig. 1. A portion of the external membrane of a villus, with a lateral 

 decidual bar. This portion of membrane is seen from its 

 foetal aspect, and in this three or four germinal centres of 

 the external cells are perceptible. 



Fig. 2. A drawing of the extremity of a villus treated with acetic acid. 

 In this villus all the parts described are distinctly seen, and 

 indicated by the same letters as in Fig. 19, Plate Y. 



Fig. 3. The extremity of a villus, with a terminal decidual bar, 

 treated with acetic acid, to show the nuclei of the decidual 

 cells in the cavity of the bar, and on the external membrane 

 of the villus. 



Fig. 4. Two tufts connected by a terminal decidual bar. 



Fig. 5. A tuft with a lateral bar passing off from its stem. 



Fig. 6. A diagram illustrating the arrangement of the placental 

 decidua. 



a. Parietal decidua. 



b. A venous sinus passing obliquely through it by a valvular 

 opening. 



c. A curling artery passing in the same direction. 



d. The lining membrane of the maternal vascular system, 

 passing in from the artery and vein lining the bag 'of the 

 placenta, and covering e e the foetal tufts, passing on to the 

 latter by two routes, first by their stems from the foetal side 

 of the cavity, and secondly by the terminal decidual bars f f 

 from the uterine side, and from one tuft to the other by the 

 lateral bar g. Throughout its whole course this membrane 

 is in contact with decidual cells, except along the stems of 

 the tufts, and the foetal side of the placenta, where the 

 decidual cells have degenerated into fibrous or areolar fibres. 

 All that portion of the decidua which is in connection with 

 the bars, villi, and tufts, is the central or functional portion 

 pf the decidua, and along with the lining membrane of the 

 maternal vascular system, or external membrane of the 

 tufts, constitutes the true maternal portion of the placenta. 



