68 MALE PRO NUCLEUS. 



ovum, and containing in a clear space at its inner extremity a nucleus, 

 has been demonstrated as the result of impregnation by Bambeke (77) 

 and Hertwig (90). There can be little doubt that this nucleus is the 

 male pronucleus, and that the pigmented streak indicates its path 

 inwards. Close to it Hertwig has shewn that another nucleus is to be 

 found, the female pronucleus, and that eventually the two join together. 

 In Amphibia the phenomena accompanying impregnation are clearly 

 of the same nature as in the Invertebrata. A precisely similar series 

 of phenomena to those in Amphibia has been shewn by Salensky 

 to take place in the Sturgeon. 



Although there is a general agreement between the most recent observers, 

 Hertwig, Fol, Selenka, Strasbtirger, c., as to the main facts connected 

 with the entrance of one spermatozoon into 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 the occurrences which succeed impregnation, according to the 

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

 nation 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 strife 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 variegcctus, 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 impregnation. 



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 



