External Factors that Influence Growth 259 



due to a difference in density or to an injurious effect of the salt. 

 Control experiments should have been made with different solu- 

 tions of such substances as sugar or urea. 1 



Frazeur tried the effects of solutions of sodium chloride on the 

 rate of regeneration of Nais. The twelve anterior segments were 

 first cut off and placed in solutions of different strengths. After 

 ten days the number of new segments that regenerated was 

 counted. The results are shown in the table : 



Sargent has studied the rate at which the process of fission takes 

 place in Dero vago. Ordinarily Dero doubles its numbers every 

 ten days. The worms were kept in solutions of sodium chloride, 

 magnesium sulphate, calcium chloride, magnesium chloride, 

 and potassium chloride. The results seem to show that the 

 rate of multiplication falls off rapidly with an increase in the 

 strengths of the solutions employed. 2 As the results stand, it 

 is not clear that they are due, solely, to the increased osmotic 

 pressure, but the salts themselves or their ions may have been 

 directly injurious. 



Loeb has shown that the new head of the marine hydroid 

 Tubularia grows longer in diluted sea-water than in pure sea 

 water, and he thinks the results must be due to a change in the 

 osmotic pressure of the solution. This might be tested by first 

 diluting the sea water and then bringing up its osmotic pressure 



1 In some recent experiment that I have carried out I found that the eggs of 

 the frog are affected both by the osmotic pressure and by the chemical substances. 



2 These results of Frazeur and of Sargent are taken from Davenport's "Ex- 

 perimental Morphology." 



