OBSERVATIONS ON UNICELLULAR FORMS 75 



increase in light intensity, and Pringsheim (1879, pp. 334, 

 367) maintains that local retardations in streaming move- 

 ment can be produced by local stimulation. Jennings 

 (1904) has shown the same to be true for Amoeba when 

 stimulated mechanically and chemically. 



After completing this part of the manuscript I had the 

 opportunity of observing the orienting reactions in Amoeba 

 proteus in detail, and also the effect of different rays on the 

 reactions. I shall insert a description of the former here; 

 the latter will be discussed in Part IV. 



In studyingorientation numerous specimens were mounted 

 under a large cover glass supported by a ring of vaseline so 

 as to give them ample room for moving about and to pre- 

 vent the solution from drying up. The specimens thus 

 enclosed could be kept in excellent condition for several 

 days. The observations were made under a compound 

 microscope situated in diffuse daylight without any screen 

 around it. Mirrors were so arranged that two horizontal 

 beams of direct sunlight were reflected upon the stage at 

 right angles to each other after passing through 8 cm. of 

 water to eliminate the heat. Specimens exposed in one 

 of these beams without any light from the substage were 

 found to direct their course in a general way from the source 

 of light. In one instance, after a slide had been exposed 

 for fifteen minutes, there were eleven specimens in one field 

 of the low power, all but two of which were moving from 

 the source of light. In another field there w^ere twelve 

 specimens; all but four of these were directed from the 

 source of light. Of these four, two were proceeding at 

 right angles to the rays and two were going toward the light. 

 In still another field containing nine specimens, seven were 

 negatively oriented, one positively and one at right angles 

 to the rays. Orientation, however, was not very precise in 

 any of the specimens. The amoebae usually took a sort 

 of zigzag course. Pseudopods were frequently seen to 

 extend toward one side for some distance, then stop as 

 though they had been checked, after which new ones were 



