THE EFFECT OF EXTERNAL INFLUENCES ON CELLS 



57 



light. The red rays are said to exercise the most powerful influence on 

 Amoebae (Harrington and Learning). In frog and triton embryos both in 

 the egg and in the young larval stage, light calls forth pronounced move- 

 ments, and in this case the blue and violet rays are most powerful (Finsen). 

 The Rhizopod Pelomyxa contracts on sudden illumination. A species of 

 Bacterium (pJiotometricum) is stimulated to active movements by light, while 

 in the dark it lies perfectly still. In the microspectrum (Fig. 33) the major- 

 ity of these Bacteria wander into the ultra red, while another collection is 

 formed in the orange and yellow (Engelmann). 



The effects on the direction of movements produced by the light are 

 designated by the term phototaxis. 



Free-living unicellular organisms, inclosed in a drop of water, collect on 

 the side of the drop turned toward the light (positive phototaxis), if the illumi- 

 nation is moderate. They flee from this side and collect on the opposite edge 

 (negative phototaxis) if the illumination is strong. Thus we have in different 



tfi:; -.: 



a B C U b /' g 



FIG. 33. The distribution of Bacteria (Bacterium photometricum) in the microspectrum of 



direct sunlight, after Engelmann. 



degrees of illumination the same difference with which we have become ac- 

 quainted under chemotaxis. In general the short-waved rays of the spectrum, 

 the blue and violet, are the most important in this directive influence. 1 



Loeb, Finsen, Adams, Yerkes, and others have described similar phenom- 

 ena among the Metazoa. Earthworms exposed to a light varying in strength 

 from 192 to 0.012 candle power exhibit negative phototaxis, which dimin- 

 ishes with the intensity of the light: in 0.011 candle power they exhibit posi- 

 tive phototaxis. In diffuse daylight the frog is positively phototactic : in 

 direct sunlight it shows at first positive, then negative phototaxis. 



The movements of the pigment cells of the retina by which their processes 

 become longer and the inner ends of the cones shorter may be cited as an unmis- 

 takable example of phototaxis (cf. Chapter XXI). Even the unconscious reflex 

 movements of the eyes, of the head and of the body as a whole, which are pro- 

 duced by stimulation of the visual cells of the retina by light may be regarded as 

 in a certain measure a kind of phototaxis (cf. page 54). 



The ultra-violet rays exercise a very marked influence on cells. On the 

 anterior portion of the eye they produce an excitation which is characterized by 



1 See note page 59. 



