OBSERVATIONS ON UNICELLULAR FORMS 1 07 



widely quoted, and it is generally assumed that they prove 

 that the sensitive portion of Euglena is located anteriorly 

 from the eye-spot, and some hold that they even prove 

 that the eye-spot does not function in light reactions at all. 

 My observations on Euglena, however, seem to indicate 

 that the portion most sensitive to light lies in close proximity 

 with the inner surface of the eye-spot, not in front of it. 



I repeated the experiment of Engelmann as follows: An 

 opaque screen containing a rectangular opening 2X3 cm. 

 was placed between the microscope and a Welsbach burner 

 as near the globe of the burner as possible. A piece of tin 

 was then hung inside the globe of the burner a few milli- 

 meters from the Welsbach mantle and so arranged that 

 one of the straight edges could be seen through the open- 

 ing in the screen. By means of the Abbe condenser that 

 portion of the mantle exposed was focused on a slide under 

 the microscope, containing Euglena deses and E. viridis (?), 

 also E. triqueter and other species. The edge of the tin 

 focused on the slide gave a strikingly sharp edge between 

 the light area and the shadow. The reactions of the 

 Euglenae were studied as they approached this edge. Both 

 low and high power were used in the observations. The 

 relation of intensity between light and shadow could be 

 regulated by manipulating the iris diaphragm connected 

 with the Abbe condenser, and the light area could easily 

 be shifted by turning the mirror. In this way it was pos- 

 sible to move the shadow of the tin over any portion of the 

 specimens while they were in motion. The Euglenae under 

 observation swam about very slowly, E. deses at the rate 

 of approximately 0.3 mm. per minute and viridis (?) not 

 much faster. Every movement and reaction could be 

 distinctly seen. I was however able to confirm Engel- 

 mann's conclusions only in part. The Euglenae generally 

 reacted before the entire body entered the shadow, and no 

 response was observed when the posterior end was shaded 

 until the shadow reached the anterior end, proving in 

 accord with Engelmann's conclusion that the anterior end 



