IMPREGNATION OF THE OVUM. 83 



by the fusion of two or more nuclei, and Strasburger at first made a 

 similar statement for Phallusia, though he has since withdrawn it. Though 

 Biitschli's statements depend, as it seems, upon a false interpretation of 

 appearances, he nevertheless arrived at a correct view with reference to 

 what occurs in impregnation. Van Beneden (78) described in the rabbit 

 the formation of the original segmentation nucleus from two nuclei, one 

 peripheral and the other central, and deduced from his observations that 

 the 'peripheral nucleus was derived from the spermatic element. It was 

 reserved for Oscar Hertwig (89) to describe in Echinus lividus the en- 

 trance of a spermatozoon into the egg and the formation from it of the 

 male pronucleus. 



The general fact that impregnation consists in the fusion 

 of the spermatozoon and ovum has now been established for 

 some forms in the majority of invertebrate groups (Arthropoda 

 and Rotifera excepted). Amongst Vertebrata also it has been 

 shewn by E. van Beneden that the first segmentation nucleus is 

 formed by the coalescence of the male and female pronucleus. 

 Calberla, and Kupffer and Benecke have demonstrated that a 

 single spermatozoon enters at first the ovum of Petromyzon. 



The contact of the spermatozoon with the egg-membrane causes in Petro- 

 myzon active movements of the protoplasm of the ovum, and a retreat 

 of the protoplasm from the membrane. 



In Amphibia the appearance of a peculiar pigmented streak 

 extending inwards from the surface of the pigmented pole of the 

 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, Strasburger, 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, 



62 



