FERTILISATION 191 



eggs which are fertilised in this way either do not divide at all 

 or go on dividing irregularly for a short time and then perish. 1 



It is supposed that the entrance of supernumerary sperrns_ 

 is prevented normally either by some mechanical means, such 

 as the development of a membrane formed after the penetration 

 of the first sperm, or else by a change in the chemical constitu- 

 tion of the ovum, occurring as the immediate result of fertilisa- 

 tion. 2 Thus, the brothers Hertwig 3 showed that in the case of 

 eggs the vitality of which had been reduced artificially (e.g. by 

 poisons), the vitelline membrane was formed so slowly after the 

 entrance of the first spermatozoon that others also were able to 

 make their way into the egg cytoplasm. On the other hand, the 

 ova of many animals in which no membrane is formed seem to 

 possess the capacity of resisting the entry of supernumerary 

 spermatozoa, and the same is apparently the case with those 

 ova which have a membrane before fertilisation, this membrane 

 being penetrated by only a single sperm. Loeb 4 has recently 

 suggested that polyspermy may be prevented by an alteration 

 in the surface tension of the egg after the entrance of the 

 spermatozoon. 



In the Mammalia fertilisation takes place usually in the 

 upper part of the Fallopian tube. 



THE HEREDITARY EFFECTS OF FERTILISATION 



The attempts that have been made to interpret the nature 

 and essence of sexual reproduction may be ranged under two 

 heads, representing the two chief theories that have been 

 elaborated (with some modifications by their respective ad- 

 herents) to explain the observed phenomena. 5 According to 



1 Wilson, loc. cit. 



2 Farmer, "On the Structural Constituents of the Nucleus," &c., Croonian 

 Lecture, Proc. Roy. Soc. B., vol. Ixxix., 1907. 



3 Hertwig, O. and R., " Beitriige zur Kenntniss der Bildung, Befruchtung 

 und Teilung des tierischen Eies," Morph. Jahr., vols. ii. and iii., 1887. 



4 Loeb, The Dynamics of Living Matter, New York, 1906. 



5 For accounts of the various theories which have been put forward con- 

 cerning the nature of fertilisation, see Wilson, loc. cit., Geddes and Thomson, 

 The Evolution of Sex, 2nd Edition, London, 1901; Weismann, The Evolution 

 Theory, English Translation, London, 1904; and Lock, Variation, &c., London, 

 1906. Further references are given in these works. 



