THE FERTILISATION OF THE EGG 177 



by influencing its motility. Spermatozoa swim with a spiral 

 motion, adhering to surfaces with which they come in contact, 

 a fact which maybe of some significance to the present question. 

 There are really two problems that arise in this connection, 

 for we have not only to account for the fact that a sperm may 

 eventually make contact with an egg, but also to explain how 

 it is that in general eggs are only fertilisable by sperm of the 

 same species. It is not necessary to suppose that the same 

 agencies are responsible for both phenomena. 



Considering first the influence of the egg upon sperm 

 motility, one has to face the possibility that the contact of 

 sperm and egg is a matter of pure chance, or more strictly, 

 that the only provision made to ensure fertilisation is the 

 synchronous ripening of the gonads in the two sexes, and the 

 prodigious fecundity of the species in animals where coitus 

 does not occur. Where there is congress of the sexes there 

 is nothing unlikely in this. What evidence is available has 

 been chiefly derived from studying the effect of egg 

 " secretions " on the sperm. In practice this amounts to 

 observing the effect exercised upon the latter when brought into 

 contact with sea water decanted from an egg-suspension, and 

 for brevity called egg-water. 



In the case of starfishes the influence of egg-secretion is 

 conclusive. Since it is highly improbable that immotile 

 spermatozoa can bring about fertilisation, and since in any 

 case activity must increase the chance that a sperm will make 

 contact with an egg enormously, the fact that immobile sperms 

 of Asterias are raised to intense activity by addition of egg- 

 water points strongly to the belief that, in these creatures 

 at any rate, the egg exercises some directive influence on the 

 sperm. In the absence of quantitative methods for studying 

 the rapidity of motion in spermatozoa, it is impossible to be 

 certain that egg-secretions have any action upon the 

 spermatozoa of forms like Arbacia and Nereis in which the 

 sperms are normally active in sea water. Some support is 

 given to the affirmative belief by observations of Loeb who 

 found that the spermatozoa of sea-urchins, which are immobile 

 but live for days in isotonic sodium chloride, may be made 



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