114 DESCRIPTION OF THE PLATES. 



ternal cells of the villus, is cast, before being ab- 

 sorbed through the internal membrane, by the 

 internal cells. This space, therefore, is the 

 cavity of a secreting follicle, the external cells 

 being the secreting epithelia, and the maternal 

 blood-vessel system the capillaries of supply. 

 This maternal portion of the villus, and its 

 cavity, correspond to the glandular cotyledons 

 of the ruminants, and the matter thrown into the 

 cavity, to the milky secretion of these organs. 



PLATE II. Fig. 21. A portion of the external membrane, with exter- 

 nal cells of the villus. 

 a Cells seen through the membrane. 

 b 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 ger- 

 minal centres of the internal cells are seen sur- 

 rounding 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 in. 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 exter- 

 nal 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 2. 



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. 



