Pressure upon the Regeneration of Gunda ulvse. 



17 



VI. Discussion. 



The results given in this paper show that for G. idvce there is a definite 

 value for the osmotic pressure of the surrounding medium, at which regenera- 

 tion of the hind end occurs most rapidly. This point lies inside the range of 

 variation of osmotic pressure to which the animals are subjected in their 

 natural environment. Above or below this point the rate of regeneration 

 diminishes, Until finally it is entirely inhibited. A similar phenomenon has 

 been obtained by Child (2) in Planaria by adding doses of alcohol, ether, and 

 potassium cyanide to his cultures, or by raising the temperature to a point 

 just below that which causes the death of the tissues. Child has ascribed this 

 change of rate of regeneration, with its final production of tailless forms, to 

 the lowering action of the external agent employed upon the " rate of action" 

 of the regenerating fragment as a whole. 



In the present case the morphological cause of the reduced rate of 

 regeneration can be localised in the parenchyma cells. These normally 

 migrate in large numbers to the region of a wound and there build up the new 

 tissues. When, however, the osmotic pressure of the surrounding medium is 

 removed from the optimum value, this migration is checked and finally 

 inhibited. In the latter case, the wound never heals, and the fragment 

 ultimately dies. In the case where the osmotic pressure is just short of 

 completely preventing the migration of the cells, the wound slowly closes 

 but no new organs are formed. This produces the so-called tailless forms. 

 The swelling that occurs at the hind end in these cases is not due to the 

 formation of tissue but to the development of large cavities in the gut or 

 pharyngeal chamber, or, in hypotonic media, to the absorption of water. 



In Planaria maculata, Curtis (3) describes the development and absorption 

 of the genital system as a regular seasonal change. G. ulves, however, is found 

 sexually mature at Plymouth all the year round. Once it has reached 

 maturity, the genital system is only re-absorbed under conditions which 

 make it necessary for the animals to use their own tissues as a source of 

 energy. These conditions are — (1) hunger, (2) regeneration after a cut 

 which has removed a fairly large proportion of the animal's body. (Doubt- 

 less, after loss of small amounts, some reduction takes place, but not enough 

 to be easily recognised.) In the regulation of G. ulvce, after a loss of some 

 of its parts, reduction and regeneration go hand in hand, and in some cases 

 where regeneration is inhibited by some external factor, e.g., high osmotic 

 pressure, reduction is inhibited to the same extent. 



As Stockard and others have already pointed out, regenerating parts have 

 a potent influence on the old body. This fact is again well illustrated in 



vol. lxxxviii. — b. c 



