270 
S. O. MAST 
In changing the direction of the rays so that the amoeba be- 
came strongly illuminated from the side as previously described, 
the distribution of the light intensity on each pseudopod as well 
as that on the entire organism became changed. Some pseudopods 
became more highly illuminated; others became shaded; on some 
surfaces the intensity was increased; on others it was decreased. 
I could not however, be quite certain that this change of intensity 
inhibited movement in any pseudopod which had already begun 
to extend, although it often appeared as though it did. But by 
referring to the figure it will be seen that no new pseudopods 
formed on the illuminated side. It must be then that orientation 
in these organisms is due to the inhibition of the formation of 
new pseudopods on the more highly illuminated side. There 
was no evidence whatever that a pseudopod with one side more 
highly illuminated than the opposite became curved owing to 
difference in rate of movement on the two sides or unequal con- 
traction or expansion. 
It may now be asked, what is the cause of the inhibition in 
the formation of pseudopods on the more highly illuminated side 
and the consequent orientation? Is it the direction of the rays 
through the organism in accord with Davenport's conclusion 
(1897, pp. 187, 210, 211); or the difference of intensity on sym- 
metrically located sensitive elements in accord with Loeb's 
theory of orientation (1906, pp. 130, 139) ; or is it changes of 
intensity? Are the different parts of an amoeba stimulated by 
light continuously in proportion to the absolute intensity on 
them as Loeb's theory demands or are they stimulated only when 
they get into such a position that changes of light intensity occur 
owing to the shadow of one part cast on another? All that can 
be said regarding these questions at present is that we are certain 
that a sudden change of light intensity may inhibit streaming 
movements; that after streaming is thus inhibited it may start 
again in course of a few minutes without any further change of 
intensity; that there is no evidence indicating that the direction 
of the rays through the tissue influences reactions; and that we 
are not certain what effect continued illumination without change 
of intensity may have on orientation since it is practically impos- 
