FERTILIZATION. 



31 



spermatozoa normally enter an ovum, but only one goes on to form a male 

 pronucleus. 



The ovum thus not only exerts an attractive influence toward spermato- 

 zoa, but it apparently exerts this influence only until the one requisite to its 

 fertilization has entered, after which it appears able to protect itself against 

 the further entrance of male elements. As to the means by which this is 

 accomplished little is known, although several theories have been advanced. 

 It may be that when the single spermatozoon necessary to accomplish 

 fertilization has entered the ovum, it sets up within the ovum such changes 

 as to destroy the attractive powers of the ovum toward other spermatozoa, or 

 as even to prevent their entrance. In the case of eggs where the spermato- 

 zoon enters through a micropyle, it has been suggested that the tail of the 



FIG. 17. Polyspermy in sea-urchin eggs treated with 0.005 per cent, nicotine solution. O. and R. 



Hertwig, Wilson. 



B, Showing ten sperm nuclei, three of which have conjugated with female pronucleus. C, Later 

 stage showing polyasters formed by union of sperm amphiasters. 



first spermatozoon remaining in the opening might effectually block the 

 entrance to other spermatozoa; or the passage of the first spermatozoon 

 might set up such mechanical or chemical changes in the canal as would 

 prevent further access. In most cases of eggs which have no vitelline mem- 

 brane previous to fertilization, such a membrane is formed immediately after 

 the entrance of the first spermatozoon, a natural inference being that this 

 membrane may prevent the entrance of any more spermatozoa. Biologists, 

 however, are inclined to discredit the view that the fertilization membrane is 

 a protection against polyspermy. 



The time and place of fertilization are matters of scientific interest and 

 practical importance. In the lower vertebrates, fishes and amphibians, the 

 female discharges the ova into the water at the breeding season and the male 

 likewise discharges the spermatozoa. The sperms swim about and come in 

 contact with and penetrate the ova shortly after they are discharged. If 

 fertilization does not occur both kinds of germ cells soon begin to disinte- 



