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 Mo 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 is the effect of 

 a stimulus which the nucleus applies to the protoplasm, and 

 which makes the protoplasm close around the nucleus. If 

 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 



