RESISTANCE TO HYPOTONIC SEA- WATER 225 



For this purpose experiments were repeated, using the technique 

 suggested but with the additional precaution of checking the electri- 

 cal resistance of the different solutions to which the worms were 

 exposed, at the beginning and at intervals during the progress of 

 the survival tests (AUee, 1929). 



This method of determining the amount of electrolytes present 

 measures all electrolytes, instead of depending on the well-known 

 relation between the amount of chlorine and the total salt concen- 

 tration in sea-water. The experiments to be reported fall into two 

 main groups: those in which the water had an initial resistance of 

 about 1,900 to 2,500 ohms, and those in which the initial resistance 

 was 5000 to 6,500 ohms. The experiments reported in 1928, as nearly 

 as can be told from chlorine titrations and survival values, belong 

 to the former level. Preliminary tests showed that with hypotonic 

 sea-water at the greater dilutions, and under the conditions of these 

 experiments, an initial difference of about 1,000 ohms is needed to 

 affect significantly the survival time of these worms. 



All the comparisons made concerning the effect of homotypically 

 conditioned medium upon the survival of Procerodes are summarized 

 in Table XXIV, which gives the results of nine separate sets of ex- 

 periments, each of which consisted of from three to six independent 

 sets of tests. In the results down to those of Experiment 71 the 

 initial resistance of the conditioned water was regulated by the 

 addition of distilled or of tap-water to the conditioned medium. In 

 the later experiments the resistivity was controlled by dialysis. 



Direct comparisons indicate that dialyzed Procerodes culture me- 

 dium is less effective than is similar dialyzed culture medium which 

 also contains the water extract of dead Procerodes worms. These 

 results are in keeping with those obtained in 1928 with the methods 

 then in use, and in all probability represent the true state of affairs.' 



In all, 640 worms were used, which were divided equally between 

 Procerodes-conditioned water and fresh water to which enough dilute 



' Castle (1928) in his work on the life-history of Planaria velaia similarly found that 

 in placing 50 small fragments of the flatworm in 10 cc. of distilled water some will sur- 

 vive. If the same mass of the same number of small fragments is placed in 200 cc. of 

 water, usually all will die. With small volumes, more than 75 per cent will survive. He 

 attributes this protective action of the mass primarily to the rapid conditioning of the 

 medium by the products from disintegrating pieces. 



