922 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



main villi for physiological interchange of materials between the embryo and 

 the maternal tissues. This portion of the chorionic vesicle with the enlarged 

 chorionic villi is known as the chorion frondosum (fig. 372D). The villi of 

 the chorion frondosum and the tissue of the decidua basalis together form 

 the placenta. The embryonic mesodermal tissues of the placenta are continuous 

 with mesoderm of the umbilical cord, and the embryonic blood vessels of 

 the placenta are directly continuous with the blood vessels of the umbilical 

 cord (fig. 372D). The placental area thus is a dual structure composed of 

 the decidua basalis or maternal placenta (placenta materna) and the chorion 

 frondosum or fetal placenta (placenta fetalis) (fig. 375E). The placental 

 area gradually expands during the early months of pregnancy until at about 

 the fifth month when it reaches its greatest relative size or about one-half the 

 internal aspect of the uterus. 



The early chorionic villi of about the fourth week of pregnancy are com- 

 posed of four constituent parts, viz.: 



( 1 ) blood capillaries which course within 



(2) the mesenchymal cells of the mesodermal core. Surrounding the in- 

 ternal core of mesenchyme is the trophectodermal layer composed 

 of an inner 



(3) cytotrophoblast, which is surrounded externally by the 



(4) syntrophoblast (fig. 376C and D). 



As development proceeds, the central core of mesenchyme with its blood 

 capillaries increases in size, and the cytotrophoblast layer of the trophectoderm 

 decreases in quantity, until, at about the fourth month, little remains of the 

 cytotrophoblast layer with the exception of a few scattered cells below the 

 syntrophoblast (fig. 376D). 



The placental villi are grouped together into groups known as cotyledons. 

 Between the cotyledons are the placental septa, which incompletely separate 

 the various cotyledons from each other. The origin of the placental septa is 

 uncertain, possibly being contributed to by both embryonic and maternal 

 tissues. Surrounding the villi within each cotyledon is a pool of maternal blood 

 which bathes the surfaces of the syntrophoblast of the villi. A hemochorial 

 relationship is in this way established (fig. 373D). 



4. Implantation in the Rhesus Monkey, Macaco mulatta 



The various stages of implantation and placental formation of the rhesus 

 monkey are shown in figure 375F-K. It is to be observed that the monkey 

 develops a primary placenta (fig. 375H and I) which later is supplemented 

 by another placenta, the secondary placenta, attached to the opposite uterine 

 wall (fig. 375J and K). Also, the embryo of the rhesus monkey, unlike the 

 human embryo, does not bury itself within the uterine mucosa, and the cho- 



