PLACENTA 



75 





The chorionic epithelium, whether by reason of its active proliferation or other- 

 wise, becomes, in its superficial lamella, converted into a continuous plasmodial 

 mass, while the cells in the central portions of the epithelial strands remain isolated 

 from one another by distinct cell-boundaries. Peters in his original memoir 

 suggested that the conversion of 

 the surface-layer of the epithe- 

 Hum into syncytium was due to 

 the action of the maternal blood 

 on it ; but it is more probable 

 that the differentiation into two 

 lamellae takes place much earlier 

 (c/. fig. 40, p. 29), as Van Beneden 

 has demonstrated for th e placenta 

 of the bat. 



The further changes leading 

 to the formation of the placenta 

 will be readily understood by 

 reference to the diagrams given 



Dies, 



FIG. 101. DIAGRAM TO ILLUSTRATE THE FIRST PHASE OF 

 THE PLACENTA. (After Peters.) 



mesoderm ; tr, trophoblast ; b.l., blood-lacuna ; 



sy, syncytium ; ca, maternal capillary ; dc, decidua. 



in figs. 101 to 103. The tropho- 

 blast- strands become invaded by 

 processes from the mesoderm 

 (fig. 103), and are thus con- 

 verted into the primary villi. 

 The villus-stems are at first 



simple and attached at their outer ends to the decidua (figs. 101 and 102), but they 

 soon become drawn out and greatly branched (figs. 102 and 103). As the mesoderm 

 extends into the epithelial strands, to form the cores of the villi, it is necessarily 

 covered by both cellular and plasmodial layers. The cellular layer becomes reduced, 

 as the villi develop, to a single layer of cells, except at their attached ends, and the 



core 



mcp 



FIG. 102. DIAGRAM TO ILLUSTRATE THE SECOND PHASE OF THE PLACENTA. (After Peters.) 



The mesodermic core has now invaded the strands of the trophoblast, and is beginning to branch. 

 mes, mesoderm; core, core of villus; &y, syncytium; mcp, endothelium of maternal capillary, ca ; 

 vs, intervillous space ; fib, fibrinous material deposited at junction of trophoblast with decidua. 



stems and branches now show the structure seen in fig. 104. In the mesodermic 

 core capillaries are seen containing nucleated blood- corpuscles from the embryo, 

 carried thither by the allantoic vessels in the abdominal stalk. The core is 

 covered by a double membrane, a cellular (Langhans' layer) and a syncytial. 



