36 Jf GENERA TION 



posed to act as a poison. The most important result of this experi- 

 ment is to show that the maximum growth does not take place in sea 

 water in which the animal is accustomed to live, but in a much more 

 dilute solution. Normal sea water contains about 3.8 per cent of 

 salts ; the maximum growth takes place in a solution containing only 

 2.2 per cent. Not only is the length of the stem greater in the latter 

 solution, but the thickness of the stem is also greater. The stem 

 is smaller in a solution containing more salt than that contained in 

 ordinary sea water. 



There is another variant in these solutions which Loeb takes into 

 account. With the increase in concentration of the solution its power 

 of absorbing oxygen decreases, but the difference is too slight to 

 affect the main result. 



Not only does the amount of salts in solution affect the osmotic 

 condition of the cells, but the salts also play a part in the metabolism 

 of the animal. As the result of a series of experiments, the details of 

 which may be here omitted, Loeb has shown that the regeneration of 

 tubularia takes place only when the salts of potassium and of magne- 

 sium are present. A very little of the potassium salt is necessary, 

 too much retards, and still more prevents regeneration. 



There must be also a certain amount of oxygen dissolved in sea 

 water in order that regeneration may take place. If a piece of the 

 stem of tubularia is cut off and one end pushed into a small tube 

 that fits the stem closely, and if the tube is then stuck into the sand 

 at the bottom of an aquarium, a hydranth develops only at the free 

 end of the piece, and none at the end in the tube. The result 

 appears to be due to the lack of oxygen. If the piece is then taken 

 from the tube, a hydranth may appear at the end that has been in 

 the tube. 



Another experiment shows the same result even more clearly. If 

 a piece of the stem is suspended freely in the water, so that its lower 

 end is almost in contact with the surface of the sand, but does not 

 quite touch it, no regeneration takes place at the lower end. This 

 result is interpreted by Loeb as due to the lack of oxygen in the 

 water near the surface of the sand. 1 



GENERAL CONSIDERATIONS 



In connection with the action of external factors on regeneration it 

 is evident that in some cases they may not be in themselves necessary 

 for the growth of a new part, yet when growth takes place they may 

 determine what sort of a part is produced. For instance, if gravity 



1 Jacobson has shown that the layer of water just above the sedimentary layer at the 

 bottom is poor in oxygen. 



