232 



THE UROGENITAL SYSTEM 



dectdua hasalis (d. serotina). The mucosa lining the rest of the uterus is differ- 

 entiated into the decidua vera {d. parietalis of Bonnet). 



Differentiation of the Trophectoderm.— The chorion is at first composed of 

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

 74). From the trophectoderm there is developed an outer syncytial layer, 

 the trophoderm (Fig. 239). This invades and destroys the maternal tissues. 

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

 Teacher) or by the blood escaping from the ruptured vessels under pressure 



M.ilcnuil i'o-5,/ Troplmhrm Vlrrbif ijaiid Truphoih rin 



Uterine g/awrf"^^ v, 

 Uterine rpitbclium 



Maternal vessel 



I ' leriiic gland 

 ji^ L Urine cpilhelinm 



Blood clot 

 Fig. 239. — Section through a human embryo of .16 mm. embedded in the uterine mucosa (semi- 

 diagrammatic after Peters) . am., Amniotic cavity; 6.^., body stalk; cd., embryonic ectoderm; e}il., 

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



(Peters), and thus blood lacuna are produced. The trophoderm thickens at inter- 

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

 (Fig. 239). The chorionic mesoderm grows out into these cords, which branch 

 profusely and become secondary or true villi (Fig. 242). During the development 

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

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

 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 vilU 

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



