84 MALE PRONUCLEUS. 



depend upon the difficulties of observation, but partly also upon the observa- 

 tions not having all been made upon the same species. Hertwig does not 

 enter into details with reference to the actual entrance of the spermatozoon 

 into the egg, but in his latest paper points out that considerable differences 

 may be observed in the occurrences which succeed impregnation, according 

 to the relative period at which this takes place. When, in Asterias, the 

 impregnation is effected about an hour after the egg is laid, and previously 

 to the formation of the polar cells, the male pronucleus appears at first to 

 exert but little influence on the protoplasm, but after the formation of the 

 second polar cell, the radial striae around it become very marked, and the 

 pronucleus rapidly grows in size. When it finally unites with the female 

 pronucleus it is equal in size to the latter. In the case when the impregna- 

 tion is deferred for four hours the male pronucleus never becomes so large 

 as the female pronucleus. With reference to the effect of the time at 

 which impregnation takes place, Asterias would seem to serve as a type. 

 Thus in Hirudinea, Mollusca, and Nematoidea impregnation normally takes 

 place before the formation of the polar bodies is completed, and the male 

 pronucleus is accordingly as large as the female. In Echinus, on the other 

 hand, where the polar bodies are formed in the ovary, the male pronucleus 

 is always small. 



Selenka, who has investigated the formation of the male pronucleus in 

 Toxopneustes variegatus, differs in certain points from Fol. He finds that 

 usually, though not always, a single spermatozoon enters the egg, and that 

 though the entrance may be effected at any part of the surface it generally 

 occurs at the point marked by a small prominence where the polar cells 

 are formed. The spermatozoon first makes its way through the mucous 

 envelope of the egg, within which it swims about, and then bores with its 

 head into the polar prominence. 



One important point has been so far only indirectly alluded 

 to, viz. the number of spermatozoa required to effect impregna- 

 tion. 



The concurrent testimony of almost all observers tends to 

 shew that one only is required for this purpose. But the 

 number of cases tested is too small to admit of satisfactory 

 generalization. 



Both Hertwig and Fol have made observations on the result 

 of the entrance into the egg of several spermatozoa. Fol finds 

 that when the impregnation has been too long delayed the 

 vitelline membrane is formed with comparative slowness, and 

 several spermatozoa are thus enabled to penetrate. Each sper- 

 matozoon forms a separate pronucleus with a surrounding star ; 

 and several male pronuclei usually fuse with the female pro- 

 nucleus. Each male pronucleus appears to exercise a repulsive 



