Page:Popular Science Monthly Volume 84.djvu/538

534 embryo and the uterine walls through which nutriment is absorbed by the embryo. And yet this difference is not a fundamental one for in different animals there are all stages of transition between these two modes of development. While in most fishes, amphibians and reptiles

 Fig. 14. Diagrams Showing the Early Development of the Human Oosperm. A, cleavage stage which has just come into the uterus; B and C, blastodermic vesicles embedded in the mucous membrane of the uterus; D, E and F, longitudinal sections of later stages, the anterior and posterior poles being marked by the axis a p. In C cavities have appeared in the ectoderm, entoderm and mesoderm. D, villi forming from the trophoblast (nutritive layer, tr); black indicates ectoderm (ect); oblique lines, entoderm; few stiples,stipples, [sic] mesoderm; V, villi; am, amnion; ys, yolk sac; n, neurenteric canal (x25). (From Keibel.)

the eggs are laid at the beginning of development and are free and independent during the whole course of ontogeny, there are certain species in each of these classes in which the development takes place within the body of the mother. Even in birds a portion of the development takes place within the body of the female before the eggs are laid, and there are mammals (monotremes, marsupials) in which the young are born in a very early and imperfect condition. These facts indicate that there is no fundamental difference between oviparity and viviparity. In the latter the union between the embryo and the mother is a nutritive but not a protoplasmic one. Blood plasma passes from one to the other by a process of soakage, and the only material influences which can affect the developing embryo are such as may be conveyed through the blood plasma and are chiefly nutritive in character. Careful studies have shown that supposed "maternal impressions" of the physical, mental, or emotional conditions of the mother upon the unborn child have no existence