THE EARLY DEVELOPMENT OF THE MAMMAL 447 



m 



FIG. 184. Diagrams illustrating the development of the villi in the human 

 placenta. A. B, After Peters; C. after Bryce. A. Chorionic mesoderm just 

 beginning to extend into the villi. B. Mesoderm invading the villi which are 

 now branched. Layer of Langhans cells forming beneath the syncytiotro- 

 phoderm. C. Continued branching of the villi, all now covered only by the 

 syncytiotrophoderm and the single layer of Langhans cells, b, Decidua basalis; 

 cb, capillaries of the decidua basalis; cv, capillaries of the villi; e, endothelium of 

 the maternal capillaries; /, fibrin deposited at the junction of the trophoderm 

 and decidua basilis; i, intervillous cavity filled with maternal blood; L, Langhans 

 cells; m, chorionic mesoderm; s, syncytiotrophoderm; t, trophoderm; v, villi; 

 vf, fixation villi. 



