Influence of Environment on Animals 197 



constantly occur. The reason for this becomes obvious when 

 we bring together mature eggs and equally mature and active 

 sperm of different famihes. When this is done no egg is, as a 

 rule, fertilized. The eggs of a sea-urchin can be fertilized by 

 sperm of their own species, or, though in smaller numbers, by 

 the sperm of other species of sea-urchins, but not by the sperm 

 of other groups of echinoderms, e.g., star-fish, brittle-stars, 

 holothurians, or crinoids, and still less by the sperm of more 

 distant groups of animals. The consensus of opinion seemed 

 to be that the spermatozoon must enter the egg through a 

 narrow opening or canal, the so-called micropyle, and that the 

 micropyle allowed only the spermatozoa of the same or of a 

 closely related species to enter the egg. 



It seemed to the writer that the cause of this limitation of 

 hybridization might be of another kind and that by a change 

 in the constitution of the sea-water it might be possible to 

 bring about heterogeneous hybridizations, which in normal 

 sea-water are impossible. This assumption proved correct. 

 Sea-water has a faintly alkaline reaction (in terms of the physi- 

 cal chemist its concentration of hydroxyl ions is about 10~^ n 

 at Pacific Grove, California, and about 10"^ n at Woods Hole, 

 Massachusetts). If we slightly raise the alkalinity of the sea- 

 water by adding to it a small but definite quantity of sodium 

 hydroxide or some other alkali, the eggs of the sea-urchin 

 can be fertilized with the sperm of widely different groups of 

 animals. In 1903 it was shown that if we add from about . 5 

 to . 8 c.c. n/10 sodium hydroxide to 50 c.c. of sea-water, the 

 eggs of Strongylocentrotus purpuratus (a sea-urchin which is 

 found on the coast of California) can be fertilized in large 

 quantities by the sperm of various kinds of star-fish, brittle- 

 stars, and holothurians; while in normal sea-water or with 

 less sodium hydroxide not a single egg of the same female could 

 be fertilized with the star-fish sperm which proved effective 

 in the hyperalkaline sea-water. The sperm of the various forms 



