232 



THE UROGENITAL SYSTEM 



plantation is supposed to occupy one day. At the point where the embryo 

 enters the mucosa a fibrin clot soon appears and eventually the opening is 

 completely closed (Fig. 239). 



The Decidual Membranes (Figs. 240 and 241).- With the increase in 

 size of the embryo and chorionic vesicle, the superficial covering layer of the 

 maternal mucosa bulges into the cavity of the uterus and forms the decidua 

 capsularis (old term, decidua reflexa). The deep layer of the mucosa 

 next the inner side of the embryo forms the anlage of the future maternal 

 placenta and is the decidua basalis (decidua serotina). The mucosa lining 



Maternal vessel Trophoderm Uterine gland Trophoderm 



__ 



Uterine gland 

 Uterine epitheli 



Maternal vessel 



Uterine gland 

 e epithelium 



Blood clot 



FIG. 239. Section through a human embryo of .16 mm. embedded in the uterine mucosa 

 (semi-diagrammatic after Peters), am., Amniotic cavity; b.s., body stalk; ect., ectoderm of 

 embryo; ent., entoderm; mes., mesoderm; y.s., yolk sac. 



the rest of the uterus is differentiated into the decidua vera (decidua parie- 

 talis of Bonnet). 



Differentiation of the Trophectoderm. -The chorion is at first com- 

 posed of an inner, mesodermal layer and an outer, epithelial layer, the 

 trophectoderm (Fig. 74). From the trophectoderm there is developed an 

 outer syncytial layer, the trophoderm (Fig. 239). This invades and de- 

 stroys the maternal tissues. In the latter large vacuoles are formed, either 

 directly by the syncytial tissue (Bryce and Teacher), or by the blood 

 escaping from the ruptured vessels under pressure (Peters), and thus 

 blood lacuna are produced. The trophoderm thickens at intervals and 



