THE REPRODUCTIVE ORGANS. 



227 



Still later, vascular loops from the allantois penetrate into the villi, 

 and the latter subdivide dichotomously to form the cotyledons of 

 the placenta. During this growth of the villi the ectodermal cells 

 covering them divide in such a way that the covering consists of a 

 double layer. The cells of the more superficial layer are not distin- 

 guishable from each other, but blend so as to form a layer of cyto- 

 plasm containing many nuclei, a structure called a syncytium (Fig. 

 204). The ectodermal cells beneath this syncytium are at first 



Fjg. 204. 



Connective tissue— -,'^ 7 ,\ )~ 



Blood-vessel- 



Ectoderm 

 of villus 



Proliferation island 



Blood-vessel with 

 nucleated 

 blood cells 



■ n 



Syncytium _JJfl§ - 

 of villus'' 



/■&>■ 



$> „*> Proliferation island 



"ft* ^f c "' tangentially 



Transverse section of a human chorionic villus at the fifth month of pregnancy. (X 300.) 



distinct, but during the later months of pregnancy they disappear, 

 leaving only the syncytium, in which there are occasional thickenings, 

 called proliferation islands (Fig. 204) ; still later, the syncytium also 

 becomes altered, its nuclei disappear, and the whole layer is con- 

 verted into a hyaline substance, " canalized fibrin " (Fig. 205). 



The placental villi come to occupy large spaces in the placenta 

 through which the maternal blood circulates, interchange of nutrient 

 and waste materials and of gases taking place between the maternal 

 and fcetal blood through the tissues of the villus. There is still 

 some doubt as to the way in which these cavernous blood-spaces are 

 formed. According to one view, they are dilatations between the 

 uterine mucosa and the foetal parts of the placenta, into which the 

 bloodvessels open. Another view is that the villi penetrate into the 

 dilated vessels of the decidua serotina. Those who hold this view 



