FEHTTLISATTON OF THE EGGS OF ANIMALS. M7 



vessel, each (jvmn becomes in a few moments surrounded by 

 a dense fringe of spermatozoa, attached to its periphery by 

 their heads, and by their movements actually causing the 

 ovum to move about. The nature of the attraction is not 

 positively known, but Pfeffer's researches on the spermatozoa 

 of plants leave little doubt that it is of a chemical nature. 

 The experiments indicate that the specific attrac- 

 tion between the germ-cells of the same species is owing to 

 the presence of specific chemical substances in each case." 

 Here it may be at once remarked that the collection of 

 spermatozoa attached by their heads to the eggs in artificial 

 fertilisation experiments is no proof whatever that the 

 spermatozoa have been attracted from a distance to the egg 

 by a substance excreted from the latter. All that we can say 

 in such a case, without further observation, is that the eggs 

 retain the spermatozoa after these have come in contact with 

 them. 



Verworn^ goes so far as to say : " The splendid and methodi- 

 cal i-esearches of Pf offer upon chemotropism had their origin 

 in observations upon the spermatozoa of forms in which 

 chemotropic relations to the egg-cell were discovered. Such 

 relations, as we now know, have analogies in almost the 

 whole of living nature and for the fertilisation of the eggs of 

 animals by spermatozoa, just as for the eggs of plants, form 

 an indispensable condition. The spermatozoon seeks the egg 

 and is guided on the right course everywhere in the living 

 world by a chemotropic action, which the metabolic products 

 of the eggs exercise upon the free-swimming spermatozoa. 

 That from the innumerable hosts of spermatozoa of the most 

 diverse animals which in many places cloud the sea, each 

 species finds its right and specific egg, a phenomenon which 

 would otherwise excite astonishment, is in tlie great majority 

 of cases a direct result of chemotropism, and easily explains 

 itself on the ground that each spermatozoon is chemotropi- 

 cally attracted by the specific substances which characterise 

 the eggs of the species concerned." It is one of the objects 

 ^ Verworii, ' Pliysiologie,' 1895, p. 425. 



