240 FOUNDATIONS OF BIOLOGY 



takes place within the oviduct, by special fluids secreted for 

 the purpose. In the highest plants, however, it will be 

 recalled that the characteristic motility of the sperm is lost 

 in the excessive specialization attendant upon gametophyte 

 reduction the sperm nucleus reaching the egg by the 

 growth of the pollen tube down through the tissues of the 

 style. 



A question of much interest is how the actual meeting of 

 the gametes is brought about. In many cases it is un- 

 doubtedly merely by chance; the random swimming of the 

 sperm sooner or later bringing one in contact with an egg. 

 In other cases the movements of the sperm seem to indicate 

 some definite attraction by the egg. It has been shown, 

 for example, that the sperm of some Mosses and Ferns are 

 attracted by exceedingly dilute solutions of cane sugar and 

 malic acid respectively, traces of which are secreted by the 

 tissues in the vicinity of the egg. Also the sperm of some of 

 the lower animals are attracted by substances eliminated by 

 the egg during maturation. In such instances there can be 

 but little doubt that chemical stimulation of the sperm by 

 specific substances plays a part in bringing the gametes to- 

 gether. This is an example of CHEMOTAXIS : a phenomenon 

 of considerable importance, especially in the behavior of 

 free-living cells. 



Once a single sperm has come into functional contact with 

 the egg, it initiates a chain of events which constitutes fer- 

 tilization. Although, as might be expected, the variations in 

 details are legion, they do not obscure the main facts. The 

 first reaction on the part of the egg is to prevent the entrance 

 of other sperm and thereby to insure a free field for the opera- 

 tions of the first arrival. In some of the lower plants this is 

 accomplished by secreting instantly a chemical substance 

 which repels other sperm. Frequently among animals a 



