FACTS AND FACTORS OF DEVELOPMENT 53 



tinguishing between different kinds of stimuli 

 is far from perfect. 



Egg cells and spermatozoa show this prop- 

 erty of sensitivity. The egg is generally in- 

 capable of locomotion, and since the results of 

 stimulation must usually be detected by move- 

 ments it is not easy to determine to what ex- 

 tent the egg is sensitive; but though the egg 

 lacks the power of locomotion, it possesses in 

 a marked degree the power of intra-cellular 

 movement of the cell contents. When a 

 spermatozoon comes into contact with the sur- 

 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. 3, 4, 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 



