232 STUDIES IN GENERAL PHYSIOLOGY 
a polyp is present; or, what is perhaps more correct, Tubu- 
laria grows in length periodically at its oral end, every 
period beginning with the formation of a new polyp, and 
ending when this organ drops off. It is therefore necessary 
that all the animals of a series of experiments which are to 
be compared with each other should be in the same phase of 
growth. Since this does not necessarily occur naturally, I 
cut off the polyps of all of the animals when a series of 
experiments was started. In all these then began a new 
period of growth, in which a polyp was first formed, and 
after which the stem grew in length (the growing part of a 
stem being situated close behind the tip of the polyp). 
I waited until the polyps had dropped off in all the speci- 
mens, and then I knew that the period of growth was at an 
end. I then compared the longitudinal growth in the indi- 
vidual specimens which had been subject to different condi- 
tions. Since growth always occurs with the formation of a 
new polyp, it follows, without further comment, that the 
concentration limits for the regeneration of the polyps are 
also the concentration limits for the growth of the Tubularian 
stem. 
3. I cut pieces having about the same length and thick- 
ness from the stems of a large number of individuals and 
distributed them equally into various dishes containing sea- 
water of different concentrations. Every vessel contained 
seven to nine animals. After eight days, in which time they 
had formed new polyps and grown vigorously, the Tubu- 
larians were removed and the amount of new growth was 
measured. The following table shows the increase in the 
linear growth of the individual Tubularians. The figures of 
the first horizontal line show the amount of salt, in per cent., 
contained in the different solutions used; in the vertical line 
under each of these figures is given the increase in the length 
of the individual Tubularians. 
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