EXPLANATION OF TIIK TLATLS. \ix 



Fiy. 21. A portion of the external membrane, with external culls of 

 the villus. 

 a. Cells seen throiij^'h the membrane. 

 h. Cells seen fnnu witliin tlie villus. 

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

 Fig. 22. Tlie extremity of a villus trt-ated with acetic acid. All the 

 parts are distinctly visible, and the germinal centres of the 

 internal cells are seen surrounding the und^ilical 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 dccidua. 



Plate VI. page 445. 



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

 decidual bar. Tliis portion of membrane is seen from its 

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

 the external cells are percei»tible. 



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 V. 



Fig. 3. The extremity of a villus, with a tenninal 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. G. A diagram illustrating the arrangement of the placental 

 decidua. 

 a. Parietiil decidua, 



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

 opening. 



c. A curling artery passing in the same direction. 



d. Tlie lining membrane of the maternal vascular system, 

 passing in from the artery and \q\\\ lining the bag of the 

 placenta, and covering c 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 // 

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

 lateral bar g. Throughout its whole course tliis mendn-ane 

 is in contact ^\^th decidual cells, except along the stems of 

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

 decidual cells have degenerated into fibr«.)us or areolar fibres. 

 All that jiortiiin of the decidua which is in contiection with 

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

 of the decidua, and along with the lining niembi-ane of the 

 maternal Aascular system, or external membrane nf the 

 tuft.s, constitutes the true malenial jwrtiou of the placeala. 



