278 



ELEMENTS OF BIOLOGY 



bloods are, however, always separated by a membrane (Fig. 190), so 

 that any substance or influence in the body of the mother can affect 

 the development of the offspring only by being in solution in the 

 maternal blood and diffusing through a membrane, obeying the 



PLACENTAL *»'>4Z.A -^^^^~=^^^~~"^ 



FOETAL 

 BLOOD VESSEL 



LAKE OF :•:; 



MATERNAL BLOOD'rh 



VEIN 



WALL OF UTERUS 



ARTERY 



Fig. 190. — Diagrammatic section through the human placenta in position. Note 

 that the blood vessels of the foetus dip into the lakes of maternal blood and that 

 they do not receive blood directly from the mother. Note also the absence of nerves 

 in the foetal portion. 



principles of membrane permeability set down in a previous chap- 

 ter (p. 32). Many erroneous ideas are current concerning the influ- 

 ence of mother on the offspring during development; some of these 

 explain birth marks as the results of fright or other emotional ex- 



