FACTS AND FACTORS OF DEVELOPMENT 



25 



that the same sort of response follows when a frog's egg is pricked by a 

 needle, thus showing that in this case the egg does not distinguish be- 

 tween the prick of the needle and that of the spermatozoon. The sper- 

 matozoon is usually a locomotor cell and it responds differently to cer- 

 tain stimuli, just as many bacteria and protozoa do; spermatozoa are 

 strongly stimulated by weak alkalies 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 em- 

 bryonic cells are usually limited to flowing movements within the 



a 



Fig. 18, a, b, c. Repulsion op Spirilla by Common Salt, a, condition immedi- 

 ately after adding crystals ; 6 and c, later stages in the reaction. 



<d> V) z, repulsion of Spirilla by distilled water. The upper drop consists of sea- 

 water containing Spirilla, the lower drop of distilled water. At x these have just been 

 united by a narrow neck ; at y and z 3 the bacteria have retreated before the distilled 

 water. (From Jennings, after Massart.) 



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 internal stimuli and by non-localized 

 external ones. Modifications of the external stimuli often lead to modi- 



