EXTERNAL FACTORS OF REGENERATION IN ANIMALS 35 



is put into a dish of water and is kept from all motion, the parts that 

 come in contact with the dish produce roots that attach themselves. 

 Even the apical end of the stem may grow out as a root, as shown in 

 Fig. 15, A. Those parts of the branch that are not in contact with 

 any solid object give rise to new hydranths. Another hydroid, Pen- 

 naria tiarella, also shows, according to Loeb, the same response to 

 contact. In this connection it is interesting to find that a growing 

 hydranth of pennaria, if brought in contact with a solid body, turns 

 away from the region of contact and bends at right angles to the body 

 which it touches. We find, once more, that a factor having an influ- 

 ence on the growth of the animal has also a similar influence on the 

 regeneration. 



.Loeb has found that if pieces of the hydroid Campanularia are cut 

 off and placed in a dish filled with sea water, all the hydranths that 

 touch the bottom of the dish are absorbed and transformed into the 

 substance of the stem. The coenosarc may creep out of the stem 

 wherever it comes in contact with the glass, and produce stolons that 

 give rise to new polyps on their upper surfaces. Loeb shows that 

 growth takes place at the end of the stolon that pushes out of the 

 perisarc, and this growing region draws the rest of the coenosarc after 

 it. If a new hydranth appears along the old piece, the coenosarc is 

 drawn towards the hydranth. 



EFFECT OF CHEMICAL CHANGES IN THE ENVIRONMENT 



Temperature, light, gravity, and contact are the most familiar kinds 

 of external physical agencies that have a direct influence upon the 

 growth of organisms. Food, though coming from the outside, yet acts 

 only after it has entered the body. Organisms that live in water may 

 be affected by the quantity and the kinds of the salts contained in the 

 water, and also by the dissolved gases. The only experiments that 

 have been made to show the influence of this last class of agents on 

 animals are those made by Loeb. He placed pieces of the stem of 

 tubularia in sea water of different degrees of concentration. After 

 eight days the pieces, that had meanwhile produced hydranths, were 

 measured. It was found that the maximum growth in length takes 

 place, not in normal sea water, but in a much diluted solution. Loeb 

 interprets this result to mean that the cells of tubularia must have a 

 certain amount of turgidity in order to grow, and this is possible so 

 long as the concentration does not pass a certain limit. This limit is 

 reached by the addition of 1.6 grams of sodium chloride to each 100 

 c.c. of sea water. With a decrease in the concentration, the cells 

 become more turgid, the maximum point corresponding to the 

 maximum amount of growth. Below this point the solution is sup- 



