150 TROPISMS 



H 2 S0 4 of a concentration as high as N/10 and that they 

 are never negatively chemotropic, not even to the highest 

 concentrations of the strongest acid. It seems to the 

 writer that Lillie's observations are more naturally ex- 

 plained on the assumption that when an acid is sufficiently 

 strong and concentrated, e.g., N/10 HN0 3 or H 2 S0 4 , it 

 will paralyze and kill the spermatozoa, and that when 

 a drop of such acid is introduced in sea water containing 

 spermatozoa, a somewhat denser ring of the organisms 

 will be formed around the surface of the drop on account 

 of this action of the acid. 



With the same method Lillie tried to prove that the 

 spermatozoa of Nereis and Arbacia are positively chemo- 

 tropic to extracts of their own eggs. 283 He proceeded as 

 follows: A suspension of Arbacia sperm, freshly made, 

 was put under a raised cover slip and a drop of the super- 

 natant sea water which had been standing over eggs (as 

 in Buller's experiments) was introduced under the cover 

 slip. Observation with the naked eye showed that around 

 this drop of egg-sea water immediately a dense ring of 

 spermatozoa formed and behind this a clear external zone 

 was formed about 1.2 to 2 mm. wide. The dense ring then 

 broke up into small agglutinated masses. In Lillie's 

 opinion the formation of this dense ring of spermatozoa 

 at the periphery of the egg-sea water is the expression of 

 a positive chemotropism of the spermatozoa for a sub- 

 stance contained in the egg-sea water, the "fertilizin." 

 He assumes that the spermatozoa near the drop of egg- 

 sea water all swim to the egg-sea water, leaving a clear 

 space behind them. While this explanation of the ring 

 formation might be true — if supported by a direct chemo- 

 tropic method like Pf effer 's — it can be shown that the ring- 

 formation is in all probability due to an entirely different 



