Influence of Environment on Animals 197 



constantly occur. The reason for this bcconics ()})vious when 

 we bring together mature eggs and equally mature antl active 

 sperm of different families. When this is done no ep:g is, as a 

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

 sperm of their o\vn 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 showTi that if we add from about 0.5 

 to 0.8 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-fi^h, l)rit tie- 

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

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

 be fertilized with the star-fish sperm which provcil ctTective 

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



