28 



Heredity and Environment 

 B 



G 



FIG. 13. A-H, successive stages in the early development of the human 

 embryo; A, blastodermic vesicle showing primitive axis in embryonic 

 area, age unknown; B, blastodermic vesicle attached to uterine wall at 

 the posterior pole, showing neural groove, age unknown ; C, later stage 

 in which the neural folds are closing and five pairs of somites have ap- 

 peared, age ten to fourteen days; D, stage of fourteen somites showing 

 enlargements of the neural folds at the anterior end which will form the 

 brain, age fourteen to sixteen days ; E and F, later stages, the latter with 

 twenty-three somites and three visceral clefts; the ear shows as a de- 



