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THE POPULAR SCIENCE MONTHLY 



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 



tr ect 



Fig. 14. Diagrams Showing the Early Development of the Hdman 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, 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 inde- 

 pendent 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 exist- 



