!40 



UROGENITAL SYSTEM 



an inner mesodermal layer and an outer epithelial layer, the trophectoderm (Fig. 

 70). From the trophectoderm there is developed an outer syncytial layer which 

 we call the trophoderm (Fig. 232) . This invades and destroys the maternal tissues. 

 In it large vacuoles are formed either directly by the syncytial tissue (Teacher and 

 Bryce) or by the blood escaping from the ruptured vessels under pressure (Peters), 

 and thus blood lacuna: are produced. The trophoderm thickens at intervals 

 and forms on the surface of the chorion solid cords of cells, the primary villi 

 (Fig. 232). The chorionic mesoderm grows out into these cords, the cords branch 



Fig. 232. — Section through an embryo of 1 mm. embedded in the uterine mucosa (semidiagrammatic 

 after Peters). Am. , amniotic cavity; i.e., blood-clot; h.s., body-stalk; ect., embryonic ectoderm; oil., 

 entoderm; mes., mesoderm; ni.11., maternal vessels; lr., trophoderm; u.e., uterine epithelium; u.g., 

 uterine glands; y.s., yolk-sac. 



profusely and become secondary, or true villi (Fig. 235). During the development 

 of the villi, the blood lacunae in the trophoderm around the villi expand, run 

 together, and produce intervillous blood spaces which surround the villi and bathe 

 the epithelium with blood. The syncytial trophoderm, from being a spongy net- 

 work, is now reduced to a continuous layer covering the outer surfaces of the villi 

 and chorion. Branches of the umbilical vessels develop in the mesoderm of the 

 chorion and villi. The mesodermal core of each villus and its branches is now 

 covered by a two-layered epithelium, an inner ectodermal layer with distinctly 



