INFLUENCE OF NUCLEUS ON BEHAVIOR OF AMCEBA. 263 



It has been thus shown that there is a difference in the general 

 movements of the segments containing and those not containing 

 a nucleus. Attention is now directed to the reactions to light 

 in such segments, especially the reactions resulting in orientation. 



ORIENTATION IN LIGHT. 



Normal Specimens. It is well known that certain species of 

 Amceba when placed in a horizontal beam of light, usually turn 

 until they are directed from the source of light and then continue 

 in a fairly direct course; that is, they orient and are negative. 

 If now the position of the source of stimulation is changed so as 

 to illuminate the organisms from the side, they again turn until 

 they are directed from the light, that is, they reorient. In the 

 form under consideration the reactions to light are marked and 

 orientation is fairly precise. 



Fragments. Fragments containing a nucleus present precisely 

 the same kind of reaction as do normal specimens when subjected 

 to similar light conditions. These parts react readily to light; 

 the movement is rapid; and the process of orientation is very 

 much like that found in intact specimens. On the other hand, 

 enucleated parts usually give no evidence whatever of orientation. 

 Experiments were made on 25 pairs of segments. In 21 of these 

 the nucleated parts oriented approximately as precisely as normal 

 individuals, but in the enucleated parts no indication of orienta- 

 tion whatever was observed except in three instances, and in 

 these there was only a mere suggestion of orientation. This 

 will be described in more detail later. 



Typical of the reactions in the 25 experiments mentioned above 

 are the responses to light of the parts shown in Fig. 3. The 

 reactions here represented, however, are somewhat more exact 

 and definite than those observed in some of the other segments. 

 In this case the enucleated fragment was the larger of the two 

 and contained the contractile vacuole. Both segments were kept 

 under the same cover-glass and were subjected to the same 

 light conditions. A 165-0. P. tungsten lamp was placed 13 cm. 

 distant from the reacting segment, and a piece of colorless glass 

 absorbed the heat waves from the lamp. A compound micro- 

 scope and a camera lucida were used. The temperature at the 



