36 HEREDITY AND ENVIRONMENT 



A B /gm^ c 



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 



