FCETAL MEMBRANES. 



589 



simple projections from the epithelial layer of the chorion and consist wholly of 

 epithelium. Very soon, however, two changes take place in these projec- 

 tions. They branch dichotomously giving rise to secondary and tertiary villi, 

 forming tree-like structures (Fig. 495). At the same time mesoderm grows 

 into each villus so that the central part of the originally solid epithelial villus is 

 replaced by connective tissue, which thus forms a core or axis. This connective 

 tissue core is at first free from blood vessels, but toward the end of the third week 

 terminals of the umbilical (allantoic) vessels grow out into the connective tissue 

 and the villus becomes vascular. Each villus now consists of a core of vascular 

 mesodermic tissue (embryonal connective tissue) covered over by trophoderm 



Syncytium 



Cellular layer 

 (of Langhans) 



Blood vessels 



Mesoderm 

 (core of villus) 



Intervillous 

 space 



FIG. 496. Section of proximal end of villus from chorion frondosum of human embryo 



of two months. Photograph. 



In the space above the villus is a mass of cells such as are invariably found among or attached to 



the villi (see text, page 594). 



(epithelium). At first the epithelium of the villus consists of distinctly outlined 

 cells. Very soon, however, the epithelium shows a differentiation into two 

 layers. The inner layer lying next to the mesoderm is called the layer of 

 Langhans or cyto-trophoderm. Its cell boundaries are distinct and its nuclei 

 frequently show mitosis. The outer covering layer consists of cells the bodies 

 of which have fused to form a syncytium the syncytial layer or plasmodi- 

 trophoderm. This is a layer of densely stained protoplasm of uneven thickness 

 (Figs. 496 and 497). It contains small nuclei which take a dark stain. As 

 this layer is constantly growing, and as these nuclei do not show mitosis, it has 

 been suggested that they probably multiply by direct division. 



