FERTILIZATION. 



39 



There seems to be no question that but one spermatozoon has to do 

 with the fertilization of a particular ovum. In Mammals only one spermato- 

 zoon normally pierces the vitelline membrane although several may penetrate 

 the zona pellucida (Fig. 22, 1) to the perivitelline space. Should more than one 

 spermatozoon enter such an egg— as, for example, in pathological polyspermy— 

 the result is an irregular formation of asters and polyasters (Fig. 24) and the 

 early death of the egg either before or soon after a few attempts at cleavage. 

 In some Insects, and in Selachians, Reptiles and Birds, a number of sperma- 

 tozoa normally enter an ovum, but only one goes on to form a male pronucleus. 



The ovum thus not only exerts an attractive influence toward spermatozoa, 

 but it apparently exerts this influence only until the one requisite to its fertiliza- 

 tion 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 



Fig. 24. — Polyspermy in sea-urchin eggs treated with 0.005 P er cen t- 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. 



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 spermatozoon enters through a micro- 

 pyle, it has been suggested that the tail of the 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 membrane 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. 



