FACTS AND FACTORS OF DEVELOPMENT 



535 



Fig. 15. A-H, successive stages in the early development of the human em- 

 bryo. 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. G, 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 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 depression at the dorsal 

 angle of the second cleft. G, 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. ) 



ence in fact, except in so far as the quality of the mother's blood may be 

 changed and may affect the child. At no time, whether before or after 

 birth, is the mother more than nurse to the child. Hereditary influences 

 are transmitted only through the egg cell and the sperm cell and these 

 influences are not affected by intra-uterine development. The principles 

 of heredity and development are the same in oviparous and in viviparous 

 animals — in fish, frogs, birds and men. 



Summary. — This is a very brief and incomplete statement of some 

 of the important stages or phases of the development of the body of man 

 or of any other vertebrate. In all cases development begins with the 



