382 THE PHYSIOLOGY OF REPRODUCTION 



over its whole extent. Hence the ccelom is small, and corre- 

 spondingly the separation of the yolk-sac and trophoblast is 

 insignificant (Fig. 78). The allantois grows out into the 

 coelom only to impinge on and invaginate the wall of the yolk- 

 sac. It never comes in contact with the outer wall of the 

 blastocyst. The part of the wall where the mesoblast is unsplit 

 is thrown into folds which fit into corresponding furrows of the 

 mucosa. Hence an avillous yolk-sac placenta is formed 

 (Selenka 1 ). The nutrition in the uterus is very primitive. 



EC mes 



-Am \ / 



s* 



Cho 



-Pro. am 



Ent 



FIG. 78. Diagram of an opossum embryo and its appendages. (From Minot.) 



All, allantois ; YJc, cavity of yolk-sac ; Cce, coelom ; Am, amnion ; Pro.am, 

 pro-amnion ; Emb, embryo ; EC, ectoderm ; Ent, entoderm ; mes, 

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



The ova contain a comparatively large supply of yolk granules 

 for the initial stages of development. As they travel along 

 the oviduct and into the uterus, they are invested with a thick 

 nutritive layer, derived 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 differentiation 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 watery fluid, composed of 



1 Selenka, Studien iiber die Entwicklungsgeschichte der Thiere, Wiesbaden. 



