330 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



epithelial cells and a core of connective-tissue cells 

 in which two or more small capillary blood-vessels 

 ramify. The epithelium of the villi undergoes great 

 alterations and may entirely disappear, to be re- 

 placed by isolated accumulations of large round 

 nuclei that stain intensely with nuclear dyes, and 

 that form protuberances on the surfaces particularly 

 of the large villi. These are called zellknoten, or 

 cell knots. Their origin and significance is doubtful. 

 In the earlier months of pregnancy the epithelial in- 

 vestment of each villus is clothed externally by a 

 continuous protoplasmic mass, called the syncytium, 

 containing small and irregularly scattered nuclei. 

 It is generally supposed that the syncytium repre- 

 sents the modified and disintegrated uterine epi- 

 thelium and is therefore of maternal origin. Some 

 embryologists affirm that in some villi there is a 

 membrane external to the syncytium which mor- 

 phologically represents the epithelial wall of the 

 uterine blood-vessels. The maternal blood, how- 

 ever, very soon breaks through the capillary spaces 

 of the uterine mucosa and enters the intervillus 

 sinuses clothed by the syncytium. The fetal cir- 

 culation is a closed system and nowhere is there a 

 direct intermingling of fetal and maternal blood. The- 

 oretically the exchange of gases, in the early stages 

 of development, takes place through (i) the epi- 

 thelial wall of the maternal capillaries; (2) the uter- 

 ine epithelial lining, probably represented by the 

 syncytium; (3) the epithelial lining of the chorion, 

 and (4) the epithelial wall of the fetal blood capilla- 

 ries. The first of these membranes is not always 



