Facts and Factors of Development 27 



there is no fundamental difference between oviparity and vivi- 

 parity. In the latter the union between the embryo and the 

 mother is a nutritive but not a protoplasmic one. Blood plasma 

 passes from one to the other by a process of soakage, and the only 

 maternal influences which can affect the developing embryo are 

 such as may be conveyed through the blood plasma and are chiefly 

 nutritive in character. Careful studies have shown that sup- 



tr ect tr - 



FIG. 12. DIAGRAMS SHOWING THE EARLY DEVELOPMENT OF THE HUMAN 

 OOSPERM. A, cleavage stage which has just come into the uterus; B and 



C, blastodermic vesicles embedded in the mucous membrane of the uterus ; 



D, E and F, longitudinal sections of later stages, the anterior and poster- 

 ior poles being marked by the axis a p. In C cavities have appeared 

 in the ectoderm, entoderm and mesoderm. D, villi forming from the 

 trophoblast (nutritive layer, tr) ; black indicates ectoderm (ect) ; oblique 

 lines, entoderm ; few stipples, mesoderm ; V, villi ; am, amnion ; ys, yolk 

 sac; n, neurenteric canal; x 25. (After Keibel.) 



