264 THE AMERICAN NATURALIST [Vol. XLIX 



fertilization of the egg of the sea urchin with the sperm 

 of a crinoid in normal sea water after both had been 

 treated with hyperalkaline sea water separately. This 

 observation is correct but finds its explanation in the as- 

 sumption that in such cases the hyperalkaline sea water 

 had not had time to diffuse from the jelly of the egg or 

 from the surface of the egg protoplasm by the time the 

 spermatozoon came in contact with it. In order to test 

 this view the writer treated the eggs of purpuratus with 

 a hyperalkaline solution of greater than the optimal con- 

 centration while the sperm was treated separately with 

 the optimal concentration (50 c.c. sea water + 0.6 c.c. 

 N/10 NaOH) and then both were mixed in a little sea 

 water in a watch glass. In such a case a large number 

 of eggs were fertilized, but while the fertilization occurred 

 nominally in normal sea water it really occurred in a 

 layer of hyperalkaline sea water surrounding the proto- 

 plasm of the egg. 



The conclusion from these experiments is that the block 

 to the entrance of the spermatozoon of Asterias into the 

 egg of purpuratus is of a rapidly reversible character, 

 consisting in some alteration of a physical property of 

 the surface. On this assumption the factor of specificity 

 consists of an agency which affects these properties of 

 the surface of the egg in the same sense as the increase in 

 the concentration of the alkali. It should be added that 

 the writer observed also that an increase of the concentra- 

 tion of Ca in the sea water acts in the same sense as an 

 increase in the alkalinity; and that if the concentration 

 of Ca is increased the increase of NaOH may be less than 

 is necessary otherwise. 



2. If the idea was correct that the factor of specificity 

 contained in the spermatozoon affected only the forces 

 acting at the surface of the egg; and that the lack of this 

 factor could be replaced by a rise in the alkalinity of the 

 sea water, it was to be expected that the reverse should 

 also be possible: namely, that a change in alkalinity or 

 the constitution of the surrounding medium should pro- 



