FCETAL NUTRITION: THE PLACENTA 417 



from the secretion of the tubal and uterine glands. Later the 

 embryos are also nourished by the primitive placental structures for 

 a period short in duration, but long enough to allow of the differentia- 

 tion of their main organs and systems. In the mucosa the surface 

 epithelium remains intact. The only change is an oedema of the 

 layers, and the sole nutritive material is a v^atery fluid, composed of 

 glandular secretion and a lymph transudate almost devoid of cells. 

 It is absorbed by the trophoblast cells, vsrhich here and there enlarge 

 to enormous "Nahrzellen" and so increase the absorbing surface. 

 After eight days the food supply becomes inadequate for the developing 

 embryos, and they are transferred to the pouch and nourished by the 

 mammary secretion. 



Ec mes 



-Pro. am 



Fig. 103. — Diagram of an opossum embryo and its appendages. (From Minot.) 



All, Allantois ; Yk, cavity of yolk-sac ; Coe, coelom ; Am, amnion ; Pro.am, 

 proamnion ; £!mb, embryo ; Sc, ectoderm ; £!nt, entoderm ; mes, meso- 

 derm ; s.t, sinus terminalis ; Cho, chorion (diplo-trophoblast). 



In Basyurus the allantois is vascular over a small area and comes 

 in contact with the diplo-trophoblast (Fig. 104). But the allantoic 

 vessels degenerate rapidly and completely, and the allantois again 

 Ues free in the coelom. In the region of the area vasculosa the wall 

 of the yolk-sac adheres to the uterine epithelium, and, as in the 

 opossum, forms a simple yolk-sac placenta. The superficial capillaries 

 of the mucosa, which are slightly dilated, are separated from the 

 vitelline vessels by the uterine epithelium and a thin layer of foetal 

 ectoderm. Through the two layers the gaseous exchange probably 

 takes place. Beyond the sinus terminalis, the non-vascular part of 

 the wall unites over an annular zone with the uterine epithelium by 

 enlarged ectodermal cells. These syncytial "Nahrzellen" are 

 phagocytic, and enclose fragments of epithehum and superficial 

 capillaries. Maternal blood is effused and lies in a space between 



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