272 THE AMERICAN NATURALIST [Vol. XLIX 



TABLE I 



eggs of the sea urchin purpuratus. In judging these 

 results the reader must keep in mind first that all these 

 experiments are made in a NaCl solution, and second, 

 that it requires a stronger influence to activate the 

 spermatozoa of the starfish which are at first not motile 

 in sea water (free from egg contents) than the sea urchin 

 spermatozoa which are from the very first very active 

 in such sea water and which may therefore be considered 

 as being at the threshold of activity in the pure NaCl 

 solution. 



If instead of the eggs themselves the supernatant NaCl 

 solution from eggs is added to the sperm it is found that 

 it requires a very much greater concentration of the 

 supernatant NaCl solution from Asterias eggs to arouse 

 the purpuratus sperm in NaCl into activity than if the 

 supernatant NaCl solution from purpuratus or from 

 franciscanus eggs is used. 



The question now arises whether the relative influence 

 of the egg on the motility of the sperm bears any relation 

 to the power of the latter to enter the egg; or in other 

 words if we can foretell which forms will hybridize by 

 observing the relative activating effect of the eggs upon 

 the spermatozoa. This does not appear to be the case 

 on the basis of our present limited experience, since the 

 activating effect of the franciscanus egg upon the sperm 

 of Asterias is just as great if not greater than that of 

 purpuratus eggs and yet Asterias sperm can enter the 



