FERTILIZATION 167 



the active sperm are attracted; when the random movements 

 of the sperm bring them within the sphere of chemical influence 

 of the egg, their movements immediately become directed 

 toward the unfertilized egg. Among some of the lower plants 

 it is known that weak solutions of malic acid and its compounds 

 attract spermatozoids; in others, solutions of cane sugar act 

 similarly (Pfeffer). It is at present doubtful, however, 

 whether in many animal eggs the control is also of a chemical 

 nature (Buller). In some forms the stimulus is certainly not 

 of a chemical sort, but is a contact stimulus. The sperm of 

 many fishes, for example, swim at random until they touch 

 some solid object, egg or other body, and from this they are 

 apparently unable to escape. According to the observations 

 of Drago the collection of the spermatozoa about an ovum is 

 unusual, and when it occurs, it is the result of agglutination. 

 It should be said that in most cases it is doubtful whether the 

 movements of the sperm really are given direction toward the 

 ovum, and what the nature of the stimulus may be, when such 

 is the case. * As we have seen, the contact between sperm and 

 egg results chiefly from the large numbers of sperm produced, 

 and from their general proximity to the egg resulting from 

 special habits of spawning, copulation, etc. 



When the ovum is naked, or surrounded by only a thin 

 vitelline membrane, the sperm apparently may enter at almost 

 any point on the surface of the egg. This is true of many forms 

 among the Medusa3, Turbellaria, Nemertea, Annelida, Echino- 

 dermata, Gasteropoda, Cephalochorda, Amphibia, and Mam- 

 malia. Entrance is usually said to be effected by the active 

 swimming movements of the spermatozoon, which force the 

 sharp acrosome, adapted to this purpose, through the limiting 

 surface of the ovum, into its superficial cytoplasm. But here 

 again extended evidence is lacking, and in many forms the egg 

 is known not to be a wholly passive recipient of the sperm, but 

 to take a considerable share in accomplishing its entrance. 

 Thus in the sea-urchin, when a sperm head approaches the egg 

 closely, the superficial cytoplasm, at the point nearest the sperm, 

 is elevated into a small cone or papilla called the attraction 



