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THE AMERICAN NATURALIST [Vol. XLIX 



sperm of Asterias will not fertilize the eggs of purpu- 

 ratus in sea water while it will do so in hyperalkaline sea 

 water (50 c.c. sea water + 0.6 c.c. 2V/10 NaOH). 



We, therefore, arrive at the conclusion that aside from 

 the physical conditions at the surface of the egg and the 

 spermatozoon the impact of the spermatozoon against 

 the egg is a prerequisite for the process of fertilization. 



von Dungern was, as far as the writer is aware, the 

 first to call attention to the fact that the egg itself causes 

 resting spermatozoa to become active, 15 but curiously 

 enough he tried to show that only foreign sperm is 

 " stimulated " in this way by the egg (which is, asF.Lillie 

 pointed out, not correct) and v. Dungern tried to explain 

 on this basis why it was not possible to fertilize the egg 

 of the sea urchin with the sperm of the starfish which 

 had at that time not yet been accomplished. 



von Dungern noticed that the egg of the sea urchin 

 " stimulates" the spermatozoon of starfish to greater 

 action and he concluded that since ''stimulation" accord- 

 ing to Jennings causes "motor reaction" whereby the 

 direction of the motile organism is changed this very 

 -tim ii la ting influence of the egg of the sea urchin upon 

 the spermatozoon of the starfish prohibited the latter 

 from getting into the egg. On the basis of the same idea 

 von Dungern was consistently led to the further con- 

 clusion that the egg exercised no "stimulating" influence 

 upon spermatozoa of its own species and that thereby the 

 spermatozoon of the same species was enabled to get 

 into the egg. A year after the appearance of von 

 Dungern's paper the writer succeeded in accomplishing 

 the hybridization of the sea urchin egg with starfish 

 sperm by a method which contradicted von Dungern's 

 theory, namely, by increasing the alkalinity of the sea 

 water whereby the spermatozoon is "stimulated" to still 

 greater activity; and on the other hand it is a common 

 experience that a sea urchin spermatozoon becomes more 

 active when it comes near an egg of its own species. 

 The writer was anxious to compare the activating 



