50 HEREDITY AND ENVIRONMENT 



face of the egg the cortical protoplasm of the 

 egg flows toward that point and may form a 

 cone or protoplasmic prominence into which 

 the sperm is received (Figs. 4, 5, EC). It is 

 an interesting fact that the same sort of re- 

 sponse follows when a frog's egg is pricked 

 by a needle, thus showing that in this case the 

 egg does not distinguish between the prick of 

 the needle and that of the spermatozoon. The 

 spermatozoon is usually a locomotor cell and 

 it responds differently to certain stimuli, just 

 as many bacteria and protozoa do; sper- 

 matozoa are strongly stimulated by weak 

 alkali and alcohol, they gather in certain 

 chemical substances and not in others, they 

 collect in great numbers around fertilizable 

 egg cells, etc. 



The movements of fertilized egg cells, 

 cleavage cells, and early embryonic cells are 

 usually limited to flowing movements within 

 the individual cells. These movements, which 

 are of a complicated nature, are of the greatest 

 significance in the differentiation of the egg 

 into the embryo ; they are caused chiefly by in- 

 ternal stimuli and by non-localized external 



