FACTS AND FACTORS OF DEVELOPMENT 57 

 19 











a 





i l _ -'_»i--/-_i^ ( _n' - ' ' ' » » I 1 















19* 19* 











b 





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<SO * OO" 











c 





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itf 2 * ,85 s 



Fig. 17. Reactions of Paramecium to Heat and Cold. At 

 a the infusoria are uniformly distributed in a trough, both 

 ends of which have a temperature of 19°; at b the infusoria 

 are shown collected at the cooler end of the trough; at c they 

 have collected at the warmer end of the trough. (From Jen- 

 nings, after Mendelssohn.) 



to such an extent as to have specialized organs 

 for producing movement its capacity for mak- 

 ing responsive movements to stimuli becomes 

 much increased. If the responses of animals 

 and plants to stimuli are of such a sort that 



