146 A. H. KEGINALD BULLEIJ. 



polleu-tubes takes the place of the chemotaxis of spermatozoa. 

 We may therefore say that actual contact of the sexual 

 elements of all plants from the liverworts onwards is brought 

 about by chemical stimuli. 



In all the above-mentioned groups of plants the oospheres 

 are fertilised in their place of origin without being set free. 

 The chemical stimulus, so far as is known, does not arise 

 directly from the oospheres. The sporuiatozoa of the vas- 

 culai- cryptogams are attracted into the archegonia by a 

 substance liberated from the cell -sap of the neck-canal-cells. 

 The pollen-tubes are guided un their tortuous way to the 

 oospheres by substances excreted by various tissues of the 

 ovary and ovules. 



On the other hand, the ova of animals are fertilised after 

 being set free from their place of origin, namely, the ovary. 

 Fertilisation takes place in the case of terrestrial animals, 

 e. g. mammals, reptiles, birds, and insects, in the oviduct, or, 

 as happens with many aquatic animals, e. g. Echinoderms, 

 many fishes, and amphibia, after the eggs have been deposited 

 in water. If, therefore, chemotaxis plays a role in bringing 

 the spermatozoa of animals into contact with the ova, the 

 source of stimulation must be looked for in a substance 

 excreted from the eggs. 



It appears to be the general opinion among zoologists that 

 chemotaxis is actually a factor in the fertilisation of aninnil 

 eggs. Thus Bergh ^ says that during an artificial fertilisation 

 experiment, e.g. in the case of the sea-urchin, " the sperma- 

 tozoa collect around the ripe eggs, probably attracted by a 

 special substance." 



Wilson,- in his latest edition of ' IMie Cell,' in dealing 

 with the union of the germ-cells, remai-ks : " There is clear 

 evidence of a definite atti-action between the germ-cells, 

 which is in some cases so marked (for example, in the polyp 

 Kenilhi) that when spermatozoa and ova are mixed in a small 



• licrgli, ' Vorlcsunycu ii!)cr allj,'{;iiiciiio Enibryoloyie,' 1S95, p. J3. 



* VVilbou, ' Tlie Cell in Dcvelopiiicut and liilieiilaiice,' 2ml cci., I'JUO, 

 p. I'JO. 



