FIG. 15. 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 appeared; age, ten 

 to fourteen days. D, stage of fourteen somites showing en- 

 largements 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. O, embryo of thirty-five somites showing eye, 

 branchial arches and limb buds. H, embryo of thirty-six 

 somites showing nasal pit, eye, branchial arches and clefts, 

 limb buds and heart. (After Keibel.) 



