FCETAL MEMBRANES. 



591 



homogeneous membrane. At points in this membrane are knob-like projections 

 composed of darkly staining nuclei. These are known as nuclear groups, or 

 proliferation islands, and probably represent the proximal portions of the large 

 cell masses already described (compare Figs. 497 and 499). 



Certain of the uterine stroma cells increase greatly in size and become the 

 decidual cells. These are large cells 30 to 100 microns and vary in shape. 

 Late in pregnancy they acquire a brownish color and give this color to the 

 superficial layer of the decidua parietalis. Each cell usually contains a single 



Syncytium 



Trophoderm 

 mass 



FIG. 498. Section c/ cuorion of human embryo of one month (9 mm.). Grosser. 



large nucleus. Some contain two or three nuclei. A few are frequently 

 multinuclear. 



Some of the chorionic villi float freely in the blood spaces of the maternal 

 placenta floating villi; others are attached to the maternal tissue fastening 

 villi. The villi are separated into larger and smaller groups or lobules by the 

 growth of connective tissue septa from the maternal placenta down into the 

 decidua basalis. These are known as placental septa, while the groups of 

 chorionic villi are known as cotyledons (Figs. 500 and 502). 



Both decidual cells and chorionic villi are important from a diagnostic 



