Embryology. 123 
Moreover this membrane, when present, is usually 
provided with one or more minute apertures, through 
which the spermatozo6n 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 
Fic. 32.—Human ovum, mature and greatly magnified. (After Hackel.) 
them, presumably by a stimulus due to some emana- 
tion therefrom?. In the mammalian ovum, however, 
these apertures are exceedingly minute, and distributed 
« 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. 
