28 



Heredity and Environment 



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 y blasto- 

 dermic 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 appeared, age ten to fourteen days; D, stage of fourteen somites showing enlarge- 

 ments 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 depression at the dorsal angle of the second cleft; 

 G, embryo, of thirty-five somites, showing eye, branchial arches and limb buds; H, em- 

 bryo >of thirty-six somites showing nasal pit, eye, branchial arches and clefts, limb buds 

 and heart. (After Keibel.) 



