352 A. J. Goldfarb 



The greater the difference in salinity between the solutions in 

 which stems had been kept and normal sea-water, the greater the 

 regeneration after the transfer. But this is exactly what might 

 have been expected from Experiment 28. Seventy-nine per cent 

 concentration or thereabouts, marks the toxic point beyond which 

 stems do not regenerate when transferred to normal sea-water. 



The percentage of salts present in sea-water determines whether 

 regeneration shall or shall not take place. An excess, or on the 

 contrary, too little salts present in the solution prevents regen- 

 eration. Whether the effects produced are the result of differences 

 in osmotic pressure or of the specific action of the salts or of both 

 of these factors was not determined. Regeneration took place on 

 the one hand in solutions diluted to ^5 per cent and on the other 

 concentrated to ^8 per cent. From these two extremes the number 

 regenerated increases to a maximum not in normal sea-water hut 

 in 75 to 20 per cent diluted sea-water. 



CONCLUSIONS AND SUMMARY 



It is extremely difficult, even approximately, to distinguish the 

 external from the internal factors in regeneration. Both kinds of 

 factors play important roles in the life history of Eudendrium 

 and other hydroids. For convenience and for purposes of study 

 each of the factors in these two series were separately considered, 

 though it should be remembered at all times that this is an arti- 

 ficial though convenient arrangement, and that these factors never 

 act singly and independently of the rest. These influences bring 

 about various reactions, only some of which may be said to be 

 adaptive. The following five factors may be said to result in 

 adaptive changes in Eudendrium.-^ 



I Gravity determines the position, at which regeneration 

 shall more frequently take place, and the direction of growth. 

 It does not determine the kind of regeneration, for with rare 

 exceptions only polyps are produced when regeneration occurs 

 at all. On erect stems oral and lateral cut ends regenerate pro- 

 fusely. On inverted stems, • regeneration is greatly stimulated, 



^^Loeb laid emphasis on but two factors, namely, light and gravity. 



