14 STUDIES IN GENERAL PHYSIOLOGY 
Finally, I have to mention the heliotropic investigations 
on Infusoria which were made along the lines mapped out 
by Sachs. ‘To bring these investigations before the reader 
I shall describe the more important observations which have 
been made on Euglena. The influence of the direction of 
the rays of light on these Infusoria was first demonstrated 
by Stahl:' 
Those individuals which did not swim about freely remained 
with their pointed posterior ends attached to the cover-glass or to 
other objects, while their free anterior ends were, according to con- 
ditions, either turned toward or away from the source of light. The 
longitudinal axes of both the motile and sessile Huglenz coincided 
as nearly as possible with the direction of the rays of light. The 
motionless ones behaved like the free-swimming ones whenever the 
direction or intensity of the light was suddenly changed, except 
that they reacted more slowly. If, for example, the glass slip was 
suddenly rotated through an angle of 180°, the position which the 
animals occupied originally with reference to the source of light 
was slowly reassumed, while the swimming individuals left their 
former path and moved in the original direction toward the light 
immediately after a change in its direction. 
Engelmann studied in Euglena the relation between the 
effect of the rays of light and their refrangibility.” After 
he had established the fact that when a drop of Euglenz is 
only partially illuminated the animals gradually accumulate 
in the lighted area, he brought the animals into a micro- 
spectrum. Here they collected on the more refrangible side 
of the spectrum. The orientation of Euglena therefore 
depends on the direction of the rays, and especially on 
that of the more refrangible ones. It must finally be men- 
tioned that the anterior ends of the Infusorie are most sen- 
sitive to light; yet the pigment spot is not, as might be 
supposed, the most sensitive, but the colorless protoplasm in 
front of this. 
Besides these direct effects of light in phenomena of 
1 Botanische Zeitung, 1880. 2 Pfliigers Archiv, Vol. XXIX (1882), 
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