62 



is also named the non-placental part of the chorion. On the other hand, the villi 

 on that part of the chorion which is in contact with the decidua placentalis increase 

 greatlv in size and complexity, and hence this part is named the chorion frondosum 

 (Fig. 28). 



Uterine vessels 



Uterine glands 



Syncytiotrophoblast 

 Cytolrophoblast 



Mesoderm Intervillous space 

 FIG. 36. Primary chorionic villi. Diagrammatic. (Modified from Bryce. 



The Placenta. The placenta connects the fetus to the uterine wall, and is the 

 organ by means of which the nutritive, respiratory, and excretory functions of the 

 fetus are carried on. It is composed of fetal and maternal portions. 



Fetal Portion. The fetal portion of the placenta consists of the villi of the 

 chorion frondosum; these branch repeatedly, and increase enormously in size. 

 These greatly ramified villi are suspended in the intervillous space, and are bathed 



Uterine glands 



Uterine vessels 



Syncytiolrophoblast 



Cytotrophoblast 



Core of mesoderm 



with fetal vessels 



Mesoderm Intervillous space 



FIG. 37. Secondary chorionic villi. Diagrammatic. (Modified from Bryce.) 



in maternal blood, which is conveyed to the space by the uterine arteries and 

 carried away by the uterine veins. A branch of an umbilical artery enters each 

 villus and ends in a capillary plexus from which the blood is drained by a tributary 

 of the umbilical vein. The vessels of the villus are surrounded by a thin layer of 

 mesoderm consisting of gelatinous connective tissue, which is covered by two 



