FACTS AND FACTORS OF DEVELOPMENT 35 



eggs are laid, and there are mammals (mono- 

 tremes) which lay eggs, while in others (mar- 

 supials) the young are born in a very 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 maternal 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 impres- 

 sions" of the physical, mental, or emotional 

 conditions of the mother upon the unborn child 

 have no existence 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 in- 

 fluences 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 



