FERTILIZATION ON PARTHENOGENESIS. 147 



forms noticed that gave any evidence of life at all; and none of 

 these reached the pluteus stage of development. 



Fertilization is the normal reaction between a ripe male and 

 female sex cell of the same species. Eggs may be led on to 

 development by artificial means, chemical or physical, but the 

 writer considers it just as improper to speak of fertilization in 

 these cases as to call eggs that have produced membranes only, 

 as a result of certain substances extracted from sperm, fertilized 

 eggs (Robertson '12). Penetration of an egg by a spermatozoon 

 is not fertilization. Certain internal conditions of each of the 

 partners is necessary for the fertilization reaction and sperm 

 may enter eggs when these conditions are not right for the 

 reaction ; but in an instance of this kind we do not have fertiliza- 

 tion. In the conditions observed above we obtain a certain 

 amount of development even though it deviates widely from the 

 normal conditions of this process. Some of the eggs start the 

 cleavage process but do not complete it; others go farther only 

 to fail in the attempt. This development results from some 

 reaction between egg and sperm since uninseminated eggs under 

 the same conditions do not cleave. The conditions for the 

 normal reaction have only been partially fulfilled and these eggs 

 have been fertilized only incompletely. We may speak of such 

 conditions as partial fertilization and thus recognize the quantita- 

 tive aspects of fertilization as we do the quantitative aspects of 

 artificial parthenogenesis. 



But why, may we ask, do eggs that have produced membranes 

 as a result of butyric acid give no evidence of fertilization what- 

 ever, even though they have been heavily inseminated with 

 sperm? Is the membrane the only barrier to fertilization? If a 

 sperm should come in contact with the egg cytoplasm or even 

 enter it could it fertilize the egg? 



Activation of the egg by the optimum exposure to butyric 

 acid results in membrane production: so also does activation by 

 sperm produce membranes. In neither case is subsequent 

 insemination by sperm effective. Wilson ('03) has shown for 

 Cerebratulus that pieces cut from fertilized eggs and dropped 

 mmediately into water containing sperm do not develop even 

 though pieces of much smaller size cut from unfertilized eggs do 



