CoGNiTic State of Energy 121 



Reasons have been advanced in another part of this work 

 for the view that animal forms have been derived from plants 

 (chap. 14). Now, while corresponding plastids or energids to 

 those above cited in plants have not yet been identified, the 

 often restricted area by which food is perceived and held, or 

 other particles rejected, amongst most protozoa, would suggest 

 that in addition to the general cell-protoplasm there are special 

 chemo tactic centers for food perception. Thus Jennings re- 

 gards the anterior end of the body in Amoeba as the specially 

 chemotactic region, and remarks: "It is sometimes said that 

 these Amoebce take food at the wrinkled posterior end. This, 

 if true at all, is certainly rare; the author has never observed 

 it, though he has seen food taken in dozens of cases. The 

 essential features of the food reaction seem to be the move- 

 ment of the Amoeha toward the food body (long continued in 

 some cases), the hollowing out of the anterior end of the Amoeha, 

 the sending forth of pseudopodia on each side of and above the 

 food, and the fusion of the free ends of the pseudopodia." 

 Amongst infusors also the same is suggested. For Jennings 

 has proved that they show, like definite plant parts, positive 

 or negative chemotropism, according to the strength or nature 

 of the solution in which they are, and also that there seems to 

 be a fairly exact location of the chemotactic area. Thus he 

 says, speaking of repellent chemical stimuli: "The direction 

 toward which the animal turns in the avoiding reaction does 

 not depend on the side of the animal that is stimulated, but 

 is determined by internal relations. The animal always turns 

 toward the aboral side. It is true that with chemical stimuli 

 the stimulation usually occurs on the oral side, so that the 

 animal turns away from the side stimulated." 



As we ascend in the animal scale, however, chemotaxis be- 

 comes located mainly or wholly in the olfactory and oral areas, 

 wliich represent a correspondingly concentrated energizing of 

 the chemotactic cells. But these areas are the first usually 

 of the sense centers that unfold in the embryo. 



Heliotactic stimulation and response are markedly shown 

 by many ciliated algoid cells. Thus, no matter whether we 



