180 Experimental Zoology 



sea urchin's sperm is thus prevented by the action of the poison, 

 but it is going beyond the evidence to extend this conclusion, as 

 has been done, to other starfish and other sea urchins without 

 further examination. 



In the sea urchin, Sphaerechinus, there is a poison in the pedicel- 

 lariae that is injurious to the spermatozoa of the starfish, but if 

 this poison exists also in the egg it is not strong enough to prevent 

 the spermatozoon of the starfish from entering, at least in those 

 species of sea urchins in which such a combination has been 

 artificially brought about. The spermatozoa of Sphaerechinus 

 itself are killed by this same poison from its own pedicellariae, but 

 a much stronger dose is required. 



It has been also found by von Dungern that extracts of the 

 eggs of Echinus, Sphaerechinus, Strongylocentrotus, or Arbacia 

 do not kill the spermatozoa of the starfish even in the strongest 

 solutions. Why, then, do not the spermatozoa of the starfish 

 readily enter the eggs of these sea urchins? Their inability to 

 enter appears to be due to another factor. The egg membrane 

 of these sea urchins agglutinizes the spermatozoa of the starfish, 

 i.e. they stick to it, and this interferes with their penetration. 

 In Sphaerechinus, however, agglutinization does not take place. 

 Why, then, does not cross-fertilization occur here ? Von Dun- 

 gern claims to have found still another substance in this sea 

 urchin's egg that excites even immature sperm to greater activity. 1 

 He believes that these exciting substances may in some cases 

 prevent cross-fertilization because they change the kind of ac- 

 tivity shown by the spermatozoa. He observed that spermato- 

 zoa that do not show rotational movement on meeting a solid, 

 or semi-solid, body may do so when excited in this way and fail 

 in consequence to penetrate. 



Von Dungern also tries to show that eggs or egg membranes 

 may contain substances that favor fertilization by its own sperm. 

 Eggs of Echinus were rubbed up and mixed with pieces of jelly 

 that had first been carefully washed. Spermatozoa were then 



1 Immature sperm excited in this way may then even fertilize their own kind 

 of egg, i.e. starfish eggs. 



