160 EDOUARD VAN BENEDEN. 



ment. The only essential condition of the construction of a 

 new animal is the presence of a material capable of de- 

 velopment." 



This was at that time the opinion of most, if not of all 

 naturalists. But so great was the authority of Johann Miiller, 

 so complete the confidence which was placed in his observa- 

 tions and conclusions, that the opinion he promulgated on 

 the permanence and division of the germinal vesicle in Ento- 

 concha caused all the affirmative statements made in previous 

 works to be called in question. 



Shortly afterwards Leydig' announced that he had con- 

 firmed in the Rotifera the results obtained by Miiller in his 

 researches on the development of Entoconcha. Mecznikow^ 

 observed the division of the germinal vesicle in the Coeci- 

 domyse and Aphides. " It is hardly possible," he writes, " to 

 call in question the generality of this fact in insects." 

 Pagenstecher^ made the same observation in the Trichinae ; 

 Leuckart* in the Oxyurides ; Keferstein^ in Leptoplana tre- 

 mellaris ; Gegenbauer^ in the Medusae, Siphonophora 

 (Corynidae, Calycophoridae, Physophoridse), Pteropoda, 

 Heteropoda, and finally in Sagitta. HackeF and Kolliker^ 

 confirmed as regards the Siphonophora the statements of 

 Gegenbauer. Finally, T myself observed the division of the 

 germinal vesicle in the transparent and easily observed e^^ 

 of Distoma Cignoides.^ Resting as much on my personal 

 observations as on those of the eminent naturalists just 

 quoted, I expressed in a hesitating form the opinion already 

 announced in a manner equally general by Leydig,^° that the 

 disappearance of the germinal vesicle is only apparent, and 

 that the embryonic development begins with the division of 



^ Tr. Leydig, " Ueber den Bau und die Systeniatisclie Stelluug der 

 Eaderthiere," ' Zeit. fiir Wiss. Zoologie,' Bd. vi, p. 102. 



^ Mecznikow, " Embryologische Studien an Insecten," 'Zeitsch. fiir 

 Wiss. Zool.,' Bd. xvi, p. 484. 



* Pagenstecher, ' I)ie Trichinen,' Leipzig, 1865. 



* Leuckart, ' Die Menschliclien Parasiten,' Bd. ii, 2e Lief., p. 322. 



* Keferstein, ' Beitrage zur Anatomie und Entwickelungsgescliichte 

 einiger Seeplanarien,' Gottingen, 1868. 



^ Gegenbauer, " Beitrage zur Naheren Kenntniss der Siphonophoren," 

 *Zeit. fiir "Wiss. Zool.,' Bd. v. 'Zur Lehre voni Generationswecksel bei 

 Medusen und Polypen,' p. 24. ' Untersuchungen iiber Pteropoden und 

 Heteropoden,' Leipzig, 1855. 'Leber die Entwiekelung der Sagitta,' 

 Halle, 1856. 



7 E. Hackel, ' Zur Entwickelungsgeschiclite der Siphonophoren,' Utrecht, 

 1869. 



8 KolUker, * Die Schwimmpolypen von Messina,' Leipzig, 1853. 

 ^ Edouard van Beneden, loc cit., p. 30. 



•" Leydig, 'Lehrbuch der Histologie,' p. 10. 



