76 



PLACENTA 



Meanwhile, as the villi become drawn out and branched, the original intercom- 

 municating blood- lacunas become expanded into the inter villous space, in which 

 the maternal blood slowly circulates and bathes the villi. The placenta at first 



FIG. 103. DIAGRAM TO ILLUSTRATE THE THIRD PHASE OP THE PLACENTA. (T. H. Bryce.) 



The mesodermic processes have further branched, and are now everywhere covered by a single layer 

 of cells (Langhans' layer) and a lamella of syncytium. At b, where a villus is attached, the cellular layer 

 retains its primitive arrangement; mes, mesoderm ; ves, ves, vessels going to villi; s?/, syncytium ; 

 L.L, Langhans' layer; a, cross-section of a villus; dec, decidua; ca, maternal capillary. 



extends over the whole chorion, but when the villi of the chorion Iseve degenerate 

 it becomes confined to the chorion frondosum. Here the villi become continually 

 more branched, and new villi are formed, until the complicated sponge-work 



L.I. 



FIG. 104. SECTION OP A VILLUS FROM AN 

 OVUM OF THE THIRD WEEK. (T. H. Bryce.) 



sy, syncytium; L.I., Langhans' layer. 



FIG. 105. SECTION OF A VILLUS FROM 

 A PLACENTA AT THE SEVENTH 

 MONTH. (T. H. Bryce.) 



of the discoidal placenta is fully developed. In the later months of pregnancy 

 the Langhans layer on the villi disappears, and the foetal capillaries are separated 

 from the maternal blood by the connective tissue of the villus and a thin lamella 

 of syncytium only (fig. 105). 



