34 



HEREDITY AND ENVIRONMENT 



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 posterior 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.) 



