STUDIES OF FERTILIZATION. 247 



spermatozoa visible in the jelly, or on the membrane of certain 

 lots of eggs none of which had fertilized ; this could not be more 

 than half of the spermatozoa in association with such eggs; and 

 other observations made immediately after insemination demon- 

 strated the high degree of motility of spermatozoa of entirely 

 barren sperm suspensions.^ 



These observations contrast in the most striking manner with 

 the fact that not a single spermatozoon can be seen in the jelly 

 of eggs fertilized with highly dilute fresh sperm suspensions, 

 where, nevertheless, nearly every egg may be fertilized. 



(b) Penetration of the egg is not solely a function of motility 

 of the spermatozoon. Penetration follows, as a matter of fact, 

 after the fertilization reaction has begun, and it is due to the 

 inception of such reaction, not the reverse as is commonly 

 assumed.^ In Nereis, as I have previously described, penetration 

 does not take place until 45 to 50 minutes after insemination 

 and the initiation of the fertilization reaction. The facts 

 described in this paper show that in Arbacia no penetration takes 

 place unless the sperm has started the fertilization reaction; if 

 this does not take place, the spermatozoon remains external, 

 however active it may be. And if it does occur the initiating 

 spermatozoon is speedily engulfed by the egg. 



(c) It is not easily understood on this theory why dilute sperm 

 suspensions should lose their fertilizing power more rapidly than 



1 Glaser's experiments (1915) would bear the interpretation that, in those cases 

 of normal insemination described by him in which fertilization does not occur except 

 in the presence of several spermatozoa for each egg, the time factor which I have 

 just described was operative. In other words that the majority of the spermatozoa 

 in question had lost their receptors. But in the absence of exact data as to age and 

 concentration of the sperm suspension, it cannot be asserted that this is the correct 

 interpietation although I obtain exactly the same results in my time series (p. 238). 

 My dilution experiments prove beyond a doubt that a single spermatozoon 

 suffices for the whole process of fertilization under optimum conditions 

 (defined on p. 233). Glaser's experiments, however, raise the question whether 

 the efficacy of heavy insemination in the case of a stale sperm suspension is 

 due to mass action, or to the survival of a small percentage of effective spermato- 

 zoa? So far as I can see this question can not be answered on the basis of our 

 present information. 



2 Spermatozoa may penetrate into unripe ovocytes in some cases, as has been 

 noted by several observers; in such a case there is no fertilization reaction. In the 

 present experiments the unfertilized eggs were not penetrated by the spermatozoa. 



