Page:Popular Science Monthly Volume 84.djvu/539

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 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 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.)

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