264 STUDIES IN GENERAL PHYSIOLOGY 



growing tissue, we not only retard or prevent these processes 

 by reducing the volume of the cells and the mechanical 

 effects of the intracellular pressure, but we reduce also the 

 irritability of the protoplasm. 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 cleav- 

 age, we at once understand why regeneration and growth 

 must be retarded or accelerated by bringing Hydroids into 

 more concentrated, or more diluted, sea-water. But if this 

 inference is right, our experiment holds good for the process 

 of cleavage not only in eggs, but in cells in general. 



The experiments which are mentioned in this paper were 

 all made on sea-urchins (Arbacia). 



The chief result of these investigations is, shortly, as fol- 

 lows. 



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 

 increase again later the amount of water, and consequently 

 the irritability of such an egg, 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. 



