158 EDOUARD VAN BENEDEN. 



kanu zwar die Moglichkeit dass der Keimfleck sich auflost 

 und der Eikern iieu entsteht, solange iiicht der directs 

 Uebergang des ersteren in den letzteren bcobachtet ist, nicht 

 ganz von der Hand gewiesen werden." 



We may, moreover, make the following remarks on the 

 arguments adduced by Hertwig : 



1. Proofs drawn from the physical and microchemicnl 

 characters of the nucleus of the egg can have only a second- 

 ary value ; but the characters common to this structure and 

 the spot of Wagner belong to all young nuclei. The only 

 conclusion from these facts is then that the nucleus of the 

 egg is a young nucleus. 



2. It does not necessarily follow from our never finding the 

 germinal spot and the nucleus of the egg existing at the same 

 moment that one is a transformation of the other ; the dis- 

 appearance of the nucleolus coincides with the appearance of 

 the nucleus of the egg ; but does it follow that the one fact 

 is caused by the other? Nevertheless, the coincidence is 

 surprising ; according to my observations on the Mammalia, 

 the formation of the central pronucleus is subsequent to 

 fecundation, and happens, consequently, long after the 

 disappearance of the germinal vesicle. 



S. I was much surprised, after reading Hertwig 's memoir, 

 to find no mention there of the " directive bodies" (Rich- 

 tungs-blaschen) which have been seen in many Echinoderms, 

 and which cannot fail to occur also in Toxopneustes. It 

 would have been interesting to know how these elements are 

 formed in Echinodermata, since, according to my observa- 

 tions on the Kabbit, one of these bodies is nothing else than 

 the germinal spot thrown off into the perivitelline liquid 

 after its previous fusion with the membrane of the germinal 

 vesicle. 



4. I cannot pass over so easily as Hertwig does the 

 difierences between the appearance presented by the nucleus 

 and the characters of the germinal spot. For my own part, 

 I do not believe that the nature of the media in which these 

 elements are respectively observed can account for the 

 marked difierences which, according to Hertwig's drawings, 

 exist between Wagner's spot and the nucleus of the egg. 

 This opinion is founded upon observations which will be 

 related further on. 



5. Hertwig has never seen any modifications produced in 

 the characters of Wagner's body during the retrograde meta- 

 morphosis of the germinal vesicle. In this I have been 

 more fortunate, not that I have studied the eggs of Toxo- 

 pneustes, but I made some observations eighteen months ago 



