FERTILIZATION 163 



fish are brought from the alkaline sea water into normal sea water 

 which contains the eggs of the sea urchin, none or only a few eggs are 

 fertilized, showing that only in the alkaline sea water does the sperm 

 of the starfish possess the qualities necessary for the fertilization of 

 the egg of the sea urchin. 



It is not so easy to decide which change must occur in the sperm of 

 the starfish in order to enable it to fertilize the egg of the sea urchin. 

 It is certain that the addition of alkali increases the energy of the mo- 

 tions of the spermatozoa of the starfish, but it is also certain that the 

 addition of bicarbonate to sea water -brings about an equal or a still 

 more powerful increase in the energy of the motions of the spermatozoa 

 of the starfish without increasing their power of fertilizing the eggs of 

 the sea urchin. At present it is generally assumed that all that is neces- 

 sary for 'the entrance of the spermatozoon into the egg is the ciliary 

 motion of the spermatozoon which brings it in contact with the egg, 

 and that the entrance of the spermatozoon into the protoplasm of the 

 egg is due to the energy of its ciliary motion. I consider it possible on 

 the basis of these observations that the ciliary motion of the spermato- 

 zoon is required only to bring spermatozoon and egg protoplasm into 

 close contact, and that the entrance of the spermatozoon into the inte- 

 rior of the egg protoplasm is due to surface tension forces. It is 

 not impossible that the conditions for this process depend upon the sur- 

 face tension between spermatozoon and sea water becoming greater 

 than the sum of surface tensions between sea water and egg, and sper- 

 matozoon and egg. In this case the egg protoplasm must spread at 

 the limit between spermatozoon and sea water. The spermatozoon is 

 thus introduced into the interior of the egg. These ideas are supported 

 by the fact that the spermatozoon of the starfish fertilizes the eggs of 

 its own species in normal sea water, and that the process is not aided 

 by making the sea water alkaline. 



It is also hardly necessary to mention the fact that the eggs of Strongy- 

 locentrotus purpuratus can be best fertilized in neutral sea water, not in 

 alkaline sea water. It is a surprising fact that in the alkaline sea water 

 in which the fertilization of the sea urchin's egg by starfish sperm succeeds 

 best, the fertilization of the same egg by sperm of their own species is 

 rendered difficult or impossible. This may be due to the fact that the 

 motility of the spermatozoa of the sea urchin is diminished by the alka- 

 line sea water. 



I have tried to fertilize the eggs of the sea urchin with the sperm of 

 Annelids and Mollusks, but thus far without success. It therefore looks 

 as if the fertilizing power of a spermatozoon were to some extent at 

 least specific. It is also possible, however, that if our idea concerning 



