252 STUDIES IN GENERAL PHYSIOLOGY 
pated, the reflexes are preserved, and only the threshold of 
stimulation for their production is raised. 
V. The brain of such an animal is regenerated in the 
course of a few weeks. 
VI. Growth and regeneration in Tubularia is, as in plants, 
dependent upon the amount of water absorbed. Growth is 
increased by an increase in the amount of water absorbed; 
while it is decreased through a diminution in the amount of 
water absorbed. Growth is practically zero in sea-water 
containing 5.1 per cent. salt, though regeneration of polyps 
is still possible; when the water contains 5.4 per cent. salt, 
regeneration also is impossible. With a decrease in the 
concentration of the sea-water, growth becomes progressively 
greater, until it attains a maximum in water containing 2.5 
per cent. salt. If the concentration is further diminished, 
growth decreases rapidly until a concentration of 1.3 per 
cent. is reached, when neither regeneration nor growth any 
longer takes place. The temperature was about 15° C. in 
these experiments. 
VII. When the pressure of oxygen is very low, regenera- 
tion no longer takes place; it is also necessary that the end 
at which regeneration is to occur be constantly surrounded by 
water containing a sufficient concentration of oxygen. 
VIII. The salt solution in which Tubularia is to regener- 
ate and grow must contain potassium and magnesium; yet 
potassium must be present only in small amounts. The 
addition of 0.33 g. of KCl to 100 c.c. of sea-water prevents 
growth; an addition of 0.6 g. to 100 cc. of sea-water pre- 
vents regeneration also. 
IX. The amount of sea-water has no noticeable effect 
upon growth in Tubularia so long as the animals are sur- 
rounded by a sufficient amount. 
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