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 KC1 to 100 c.c. of sea-water prevents 

 growth ; an addition of 0.6 g. to 100 c.c. 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. f 



