STUDY OF FERTILIZATION 423 



hydroxyl ions, dilution of sea-water, &c. It is exceedingly- 

 difficult to apply the chemical conceptions of Loeb or of 

 Lillie to such facts. It is obvious that the self-sterility of the 

 gametes of Ciona can also be analysed by a similar physical 

 argument to hybridization. 



Under normal conditions only one spermatozoon enters an 

 egg. In view of the very large number of spermatozoa which 

 may be in the immediate vicinity of the egg-surface at the 

 moment of fertilization, it is almost inconceivable that any 

 chemical change could be set up, and carried to a conclusion 

 between the time that two successive spermatozoa touch the 

 egg. Neither Lillie nor Loeb has offered what would seem to 

 be a reasonable explanation of monospermic fertilization. 

 Once more, the facts appear to be amenable to physical treat- 

 ment. Assuming that the rate at which an electrical change 

 can travel round the egg is approximately that at which 

 it travels along a piece of smooth muscle then within 0-00001 sec. 

 after the effective spermatozoon has made its contact with the 

 egg, no other spermatozoon will have any effect : if, however, 

 the eggs are treated in such a way as to reduce the rate of pro- 

 pagation of an electrical disturbance, e. g. by incomplete 

 anaesthesia (cf. nerve or muscle), then the wave will not pass 

 completely over the egg before other spermatozoa can effect 

 contact with unaffected portions of the egg-surface, and poly- 

 spermy will result. Hertwig (17) showed that unfertilized eggs 

 treated with chloral hydrate and other anaesthetics were 

 markedly polyspermic. 



The first visible sign that fertilization has occurred, is at 

 the surface of the egg. In the case of the echinoderm or annelid 

 egg, fertilization is attended by the formation of a ' fertiliza- 

 tion membrane '. It must be remembered, however, that the 

 essential change at the egg-surface is completed long before 

 any visible change is possible. What is the nature of the 

 fertilization membrane '? In the case of the egg of Nereis 

 it seems certain (Lillie, 20) that this membrane is the vitelline 

 membrane of the unfertilized egg, which is pushed away from 

 the egg-surface by the disintegration and hydration of the 



