FACTS AND FACTORS OF DEVELOPMENT 49 



B C D E F 9 



FIG. 17. DISTRIBUTION OF BACTERIA \~$ THE SPECTRUM. The 

 largest group is in the ultra- red at the left; the next largest 

 group is in the yellow-orange close to the line D. (From Jen- 

 nings, after Engelmann.) 



not to be concluded because organisms respond 

 differently to certain stimuli that they are 

 therefore capable of distinguishing between 

 all kinds of stimuli, for this is certainly not 

 true. Even in adult men the capacity of dis- 

 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- 



