ORGANIZATION AND GROWTH 235 
tween growth and the amount of water in the solution was in 
all points essentially the same as detailed here. We found in 
a preceding chapter that the polyp is regenerated later in a 
very highly concentrated or a very dilute solution than in a 
solution the concentration of which lies between these two 
extremes. We have therefore to determine in how far this 
circumstance compels us to make corrections in the experi- 
mental results given above. Between the concentrations 
4.4-2.5 per cent. the difference in time between the forma- 
tion of polyps in the different concentrations is so slight 
that they need not be considered. So far as the very con- 
centrated solutions, 5.1 and 4.6 per cent., are concerned, I 
have made experiments which I have continued for weeks 
and months, and have found that the absolute increase in 
length during this time is practically zero, even though the 
animals formed new polyps repeatedly during this time. 
4. I have not made any measurements on the increase in 
the thickness of Tubularia. Yet the effect of the concen- 
tration of the salt solution upon the diameter of the newly 
formed stem was very apparent even without measurements. 
The new stems formed in the more concentrated solutions. 
Those in which about 1 g. of NaCl had been added to each 
100 c.c. of sea-water were markedly thinner than the old ones 
which had been grown in ordinary sea-water. On the other 
hand, in the diluted sea-water the thickness of the new stems 
was not only not less than that of the old stems, but even 
greater. 
5. With an increase in the concentration of a salt solu- 
tion the amount of oxygen dissolved in it decreases, and, as 
we shall see later that this is an important factor in regenera- 
tion and growth, we must determine whether differences in 
concentration influence the growth of Tubularia through their 
effect upon the amount of oxygen dissolved in the solutions 
and, if so, how much. The one direct measurement of the 
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