GERMINAL VESICLE AND FHIST EMBRYONIC NUCLEUS. 161 



this element. I expressed myself iu the following words : 

 "En resume, je considere non eomme demontre, mais comme 

 tres-probahle, que la vesicule germiiiative se divise au lieu 

 de disparaitre " (loc. cit., p. 244). 



I supported this proposition by various more or less plau- 

 sible considerations, and iu particular, I pointed out that the 

 disappearance of the vesicle had never been observed, no 

 direct proof of this disappearance having been given. 



I should add that the contrary proposition did not, any 

 more than this, rest upon direct observation ; no one of the 

 authors who maintained the permanence of the vesicle rested 

 his opinion upon any other considerations than upon a nega- 

 tive fact. The vesicle is asserted not to disappear' because 

 no egg has ever been observed entirely devoid of a nucleus. 



Some observers have been inclined to trace a genetic bond 

 between the nucleus of the impregnated egg and the * spot ' 

 of Wagner. Leydig^ has expressed this opinion so far as 

 regards the eggs of Piscicola ; von Baer^ with respect to the 

 development of a sea urchin. Bischoff ^ derives from the 

 germinal spot of the rabbit, not only the directive bodies but 

 also the nucleus which is found in the egg after impregna- 

 tion. But BischofF confesses that it is in his eyes, a pure 

 hypothesis ', he moreover, abandoned it shortly afterwards,* 

 and in his later works, has declared it to be untenable. 

 Quite recently, FoP has found in the ripe egg of the Medusae 

 a nucleus, with respect to which he feels some doubt, not 

 knowing whether it is the modified germinal vesicle or the 

 spot of Wagner. Finally, Hertwig has just given new credit 

 to this tottering opinion by the publication of his recent 

 memoir, which cannot fail to produce a great effect. 



A short time after the publication of my memoir on the 

 composition and significance of the e^^, two publications 

 appeared in Germany, which are of the highest importance 

 with respect to the question now occupying our attention. 



Oellacher^ established the fact that in the matured ovum 

 of the salmon the germinal vesicle reaches the surface of the 

 germ and opens itself into the space which exists at that 



^ Leydig, *' Zur Anatomic von Piscicola gcometrica," ' Zeit. fiir Wiss. 

 Zool./ Bd. i. 



' K. E. von Baer, " Neue Untersuchungen iiber die Eatwickelung der 

 Tbiere," 'Froriep's Neue Notizen,' Bd. xxxix, p. 38. 



^ Bischoff, 'Entwiclielungsgescliichte des Kanincliencies,' 1812. 



■• Bischoft', ' Entwickeluugsgeschiclite des Ilundeies,' 1845. 



* Fol, "Die erste Entwickeiung des Geryonideneies," ' Jenaische Zeit- 

 schrift,' Bd. vii, p. 474. 



« Oeilaclier, "Beitiage zur Geschichte des Keimblaschens im Wirbel- 

 tbierei," ' Archiv. fiir Mikrosk. Anat.,' Bd. viii. 



VOL. XVI. NEW SER. L 



