EXPERIMENTAL ZOOLOGY 183 



some eggs may develop without spermatozoa. This fact gave 

 color to the belief that, even in eggs which in nature must 

 always be fertilized, parthenogenesis might be accomplished by 

 artificial means, and this has been found to be the case. 



Briefly, it has been discovered that by subjecting the eggs 

 of some animals to very dilute solutions of salts, acids and 

 other substances, development, hardly to be distinguished from 

 the normal, ensues, though unfortunately no one has yet suc- 

 ceeded in rearing to an adult state the forms thus started 

 upon their life cycle. 



Experiments such as these throw a light upon the prob- 

 lem of fertilization which no mere observation of the stages 

 in normal fertilization and parthenogenesis could ever give. 

 They lend support to the hypothesis that the spermatozoon 

 initiates development by bringing into the egg a minute quan- 

 tity of some substance which, on being liberated within the 

 protoplasm, furnishes the necessary stimulus to development. 

 Could we isolate from the spermatozoon a substance which, 

 when injected into the egg, would cause development exactly 

 like the normal, we should probably have as good ground 

 for such an hypothesis as we could hope to secure. It would 

 enable us to speak more precisely of the stimulus to develop- 

 ment and to remove it from the intangibility which leads us 

 to discuss so many vital phenomena in indefinite and mean- 

 ingless terms. Work of this character is now being carried 

 on by so many investigators that we may hope for a much 

 more satisfactory analysis of the stimulus by which individual 

 development is begun than has yet been possible, even though 

 the facts should bring their inevitable results in the opening 

 of many problems entirely new. 



Again, to continue our illustrations from embryology,, 

 one of the old disputes in embryologv' was over the question of 

 "preformation" or "epigenesis." Is the organism already 

 formed within the germ like the bud of a plant, and does the 

 development consist merely in an unfolding of what is already 



