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 réle 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. 
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