Embryology. 123 



Moreover this membrane, when present, is usually 

 provided with one or more minute apertures, through 

 which the spermatozoon passes when fertilizing the 

 ovum. It is remarkable that the spermatozoa know, 

 so to speak, of the existence of these gate-ways, 

 their snake-like movements being directed towards 



FlG. 33. Human ovum, mature and greatly magnified. (After Hackel.) 



them, presumably by a stimulus due to some emana- 

 tion therefrom 1 . In the mammalian ovum, however, 

 these apertures are exceedingly minute, and distributed 



1 The spermatozooids of certain plants can be strongly attracted 

 towards a pipette which is filled with malic acid crowding around and 

 into it with avidity. 



