254 LOEB. [Vol. VII. 



water were in the two-cell stage, whilst none of the eggs in the 

 other solutions were yet segmented ; in part a the first &gg 

 was segmented at 10.55 ; i"^ ^ the first segmentation took 

 place at 11.45, nearly an hour later than in normal sea-water; 

 and in c no segmentation took place at all. That the amount 

 of water and the intracellular pressure in these experiments, 

 varied with the concentration could be seen from the form of 

 the cleavage spheres. In normal sea-water, and still more in 

 sea-water which was a little diluted by the addition of 10 to 20 

 per cent of fresh water, the first two cleavage spheres were 

 nearly perfect hemispheres. In sea-water of higher concentra- 

 tion the first two cleavage spheres became ellipsoidal in shape, 

 approaching more the sphere the higher the concentration was. 

 When I added more than 2 g. NaCl to 100 ccm. sea-water, in a 

 few hours plasmolysis took place, and the surface of the proto- 

 plasm began to shrink irregularly. But by bringing the eggs 

 back into normal sea-water the normal form was restored in a 

 few minutes. 



2. Further investigations concerning this subject led me to 

 another series of facts, which, as I believe, give the physiological 

 explanation of some of the phenomena of cleavage. In my inves- 

 tigations concerning the regeneration and growth of hydroids, I 

 found that a salt solution which is just concentrated enough to 

 prevent regeneration and growth by no means kills the hydroids, 

 or even annihilates the power of growth and regeneration. 

 Hydroids which had been in such a solution for several days 

 when brought back into normal sea-water began to regenerate 

 and to grow. When I made the same experiments on fertilized 

 eggs, the results were the same. A salt solution which is just 

 concentrated enough to prevent segmentation does not annihilate 

 the power of segmentation at once. But when I brought such 

 eggs back into normal sea-water, I found that the manner of seg- 

 mentation changes in a remarkable way, according to the time 

 the eggs had been in the concentrated sea-water. 



3. I fertilized eggs of sea-urchins at 9.30 in the morning, and 

 at 9.43 a part of these eggs were put into sea-water to which 

 2 g. NaCl to 100 ccm. had been added. The rest of the eggs 

 remained in normal sea-water. I will call the sea-water to which 

 2 g. NaCl to 100 ccm. had been added the concentrated solution, 

 and the eggs which had been exposed to it the plasmolyzed 



