448 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



surface of the egg of a number of irregular clear blister-like protuber- 

 ances which rapidly increase in number and extent, finally covering 

 the surface of the egg with a clear hyaline layer, which is designated 

 the fertilization-membrane (Fig. 27). This was lacking in the artifi- 

 cially fertilized egg. 



The imperfect character of the imitation of fertilization which was 

 thus achieved led Loeb to form the supposition that the osmotic method 

 of inducing fertilization only accomplished a part of the effects ini- 

 tiated by the spermatozoon, which he inferred carried into the egg 

 agencies not only capable of starting the processes initiated by the 

 hypertonic sea-water but also processes which the osmotic method 

 did not suffice to initiate. This supposition was confirmed by the 

 discovery of a series of agents capable of inducing Membrane -forma- 

 tion in the sea-urchin egg. 



1. 2. 



FIG. 27. 1, unfertilized egg of the Sea-urchin (Strongylocentrotus purpuratus) sur- 

 rounded by spermatozoa; 2, the same egg about two minutes later, after the entrance 

 of the spermatozoon and the formation of the fertilization-membrane. (After Loeb.) 



It was found that if mature sea-urchin eggs were introduced for a few 

 minutes into sea-water to which a small proportion of a certain sample 

 of Ethyl Acetate had been added, and then returned to normal sea-water, 

 all of the eggs promptly formed a fertilization-membrane differing in 

 no perceptible degree from the membranes formed in normal fertili- 

 zation. Other esters failed to yield any comparable result, and an 

 examination of the ethyl acetate employed in the original experiment 

 showed that it had undergone hydrolysis and contained free ethyl 

 alcohol and acetic acid. This led to an investigation of the behavior 

 of the eggs in sea- water containing added alcohols and acids and to the 

 discovery that the effect originally obtained with impure ethyl acetate 

 was due to the Acetic Acid which it contained. It was found that all 

 of the monobasic fatty acids which are soluble in sea-water, namely 

 formic acid, acetic acid, propionic acid, butyric acid, valerianic acid 

 and so forth, will induce membrane formation in 100 per cent, of 

 mature eggs if they are exposed for a brief period to the action of the 

 sea- water containing the fatty acid. The formation of the membrane 



