MATURATION AND IMPREGNATION OF THE OVUM. 543 



the egg, the formation of the male pronucleus, and its fusion 

 with the female pronucleus, there still exist differences of detail 

 in the different descriptions which partly, no doubt, 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 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 impregnation 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 Nematodes impregna- 

 tion 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 cell was 

 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. The head 

 of the spermatozoon on entering the egg becomes enveloped by 

 the superficial protoplasm, and travels inward with its envelope, 

 while the tail remains outside. As Fol has described, a delicate 

 membrane becomes formed shortly after the entrance of the 



