262 LOEB. 



long since, the orientation of the plane of division, and as Hert- 

 wig believes, in such a way that the longitudinal axis of the 

 Kernspindel is put in the longest diameter of the cell. Therefore 

 we ought to expect that, within certain limits, with increasing 

 intracellular pressure, Sachs's law of the rectangular division of 

 the plains of cleavage would become more obvious. I found, 

 indeed, that in normal, or still more in somewhat diluted, sea- 

 water, where the turgor, and consequently the flattening of the 

 cleavage spheres, was the greatest, Sachs's law was the most 

 exactly realized. Therefore this geometrical regularity in the seg- 

 mentation of the nucleus which is so striking under normal con- 

 ditions must disappear at once if the protoplasm does not take 

 part in segmentation. 



9. Our observations concerning the dependence of irritability 

 of the protoplasm upon the water contained in the tissues add 

 one more fact to those given already to explain the importance 

 of water for all processes of growth and development. If we 

 reduce the amount of water in a regenerating or growing tissue, 

 we not only retard or prevent these processes by reducing the 

 volume of the cells and the mechanical effects of the intra- 

 cellular pressure, but we reduce also the irritability of the pro- 

 toplasm. This irritability, as we saw, plays an important role 

 in the process of cleavage, and as regeneration and growth is a 

 function of processes of cleavage we at once understand why re- 

 generation and growth must be retarded or accelerated by bring- 

 ing hydroids in more concentrated, or more diluted, sea-water. 

 But if this inference is right, our experiment not only holds good 

 for the process of cleavage in eggs, but in cells in general. 



The experiments which are mentioned in this paper were all 

 made on sea-urchins {Arbacia). The other experiments will be 

 published later. 



The chief result of these investigations is, shortly, as follows : 



If we reduce the irritability of the protoplasm of the egg by 

 reducing the amount of water contained in it, the nucleus can 

 segment without segmentation of the protoplasm. If we in- 

 crease again later the amount of water, and consequently the 

 irritability of such an Q.gg, the protoplasm at once divides into 

 about as many cleavage spheres as there are nuclei pre-formed. 

 The segmentation of the protoplasm in the egg, and probably 

 in every cell, is only the effect of a stimulus exercised as a rule 

 by the nuclei. 



