262 STUDIES IN GENERAL PHYSIOLOGY 
could make them negatively heliotropic. The same was the 
case in Copepods, only the absolute figures differ, as was to 
be expected. By bringing living tissues into a solution of 
higher concentration, we reduce the irritability by reducing 
the amount of water contained in them. By reduction 
of irritability we mean that the effect determined by the 
same cause is quantitatively less. That explains how the 
segmentation of the protoplasm is generally determined, 
why in a solution of a certain concentration no segmenta- 
tion of the protoplasm takes place, and why when brought 
back into normal sea-water the protoplasm segments at 
once into about as many spheres as there are nuclei pre- 
formed. The segmentation of the protoplasm ts the effect of 
a stimulus which the nucleus applies to the protoplasm, and 
which makes the protoplasm close around the nucleus. Tf 
we bring the fertilized egg in the concentrated salt solution 
(2 g. of NaCl to 100 c.c. of sea-water), the nucleus divides, 
and every nucleus applies the stimulus to the protoplasm 
with which it is in contact. But the protoplasm of the egg, 
on account of its containing too little water, is in the condi- 
tion of a cooled-off muscle, which does not answer to the 
stimulation of the nerve, and no segmentation of the proto- 
plasm takes place. But as soon as we bring the egg back 
into normal sea-water, the protoplasm takes up water very 
fast and regains its irritability ; and now, of course, it answers 
to the stimuli from the nuclei, and closes around every 
nucleus of segments. If we add a smaller dose of NaCl— 
namely 1.3 g. of NaCl to 100 c.c. of sea-water—the irrita- 
bility is only a little less than it is normally, and the whole 
effect is that the reactions of the protoplasm are somewhat 
slower and retarded. Of what kind the stimulus is, and from 
which part of the nucleus it is exercised, we cannot tell. From 
other facts I am inclined to believe that this stimulus is a 
chemical one, and caused by certain substances produced in 
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