FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 



921 



broken down to form liquefied areas, the embryotroph. It is possible that this 

 liquefied material is assimilated by the syntrophoblast and passed inward to 

 the developing germ disc. If this histological material thus is utilized it forms 

 a source of nutrition, and it may be called histotrophic nutrition. 



c. Formation of the Placenta 



As the developing chorionic vesicle grows within the endometrium of the 

 uterus, the uterine mucosa expands over the growing vesicle (fig. 377A and B ) . 

 That part of the endometrial tissue overlying the chorionic vesicle is called 

 the decidua capsularis (fig. 377A), and the portion of the endometrial 

 lining of the uterus not concerned with the enclosure of the chorionic vesicle 

 is called the decidua vera or decidua parietalis. The part of the endometrium 

 lying between the muscle tissue of the uterine wall and the enlarging villi 

 (fig. 372C and D) of the chorionic vesicle is the decidua basalis (fig. 377A). 



At first chorionic villi are developed over the entire chorionic vesicle (fig. 

 372B), but as development goes on the villi in relation to the decidua parietalis 

 are resorbed gradually to form a smooth area of the chorion, the chorion laeve 

 (fig. 372D). Finally, only those villi in relation to the decidua basalis remain 

 (fig. 372D). The villi within the decidua basalis enlarge and become the 



SYNTROPHOBLAST 



TROPHOBLASTIC 

 LACUNAE 



CYTOTROPHOBLAST 



EXTRA - EMBRYONIC 

 MESODERM 





MESENCHYMAL CELLS 



BRUSH BORDER 



CYTOTROPHOBLAST 

 (LANGHAN'S LAYER) 



SYNTROPHOBLAST --j^, 0^ / ^_,-i, <«; ^-«> ;^» 



„, -BLOOD VESSEL ■* -^ \ V r^ ^^r""^ -^^ --^^'^t. 

 HOFBAUER CELL— S ^ ^ .^ ) >^,» 5u^« 8 - 



CELLS OF 

 REGRESSING 

 CYTOTROPHOBLAST 

 C LAYER 



CAPILLARY 



Fig. 376. Structure of villi in human chorionic vesicle. (A) Primary villus. (B) 

 Secondary villus. (C) Villus from chorion at about 4 weeks. (D) Villus at about 

 14 weeks. Observe gradual disappearance of cytotrophoblast. 



