54 PROFESSOR VAN BENEDEN. 



" median lens " of ray eggs, it appears to be absent in other 

 Osseous Fishes. It even appears, according to some observers, 

 that only the peripheral welt exists at iirst, and that secondarily 

 by growth from the periphery towards the centre the intermediate 

 layer becomes complete beneath the blastodisc. 



Kupffer has put forward the opinion that his "nuclear zone " 

 is something quite distinct from the " feuillet mucpieux" of Lere- 

 boullet. It is clear from my observations that the two 

 authors have [had under their observations simply difl'erent 

 parts of one and the same layer, the extent and characters of 

 which vary in all probability from one genus to another. It is 

 this same layer which ffillaclier described under the name of 'vitel- 

 line' membrane. This excellent observer has been unfortunate in 

 the choice of a name for the layer and has been led into 

 error in assuming that the cells which he found therein at the 

 conclusion of segmentation are cells derived from the blastoderm, 

 having fallen from the roof of the germinal cavity. 



Klein not only recognised this same layer in the Trout, but 

 he demonstrated that cell-nuclei appear in it in great quantity 

 towards the end of the period of segmentation. He gives to 

 this layer the name of parablast. His, who has seen the same 

 objects in the Salmon, gives to this intermediate layer the name 

 of Hindenschlcht, the peripheral welt is called by him Keinuvall, 

 and the cells observed therein at the conclusion of cleavage are 

 designated by him Parablastisch or Nebeukeiin Zcllen. 



III. At no period of development have I found any trace of 

 that cavity which Van Bambeke calls the " caviie de segmenta- 

 tion.''^ Kupffer, who was not able to demonstrate any such 

 cavity in living eggs, found it in eggs of Gobius niger, Mhich 

 had been previously hardened in a solution of sulphuric acid. 

 LerebouUet found, when he made the eggs of the Perch and 

 the Pike coagulate, that the blastoderm, is at a given moment, 

 " a hollow vesicle.^^ Van Bambeke himself could only persuade 

 himself of the existence of his " segmentation cavity'^ by the ex- 

 amination of sections cut from hardened eggs. This cavity has 

 not been asserted to exist in the Trout by Reineck, nor by 

 Weil, nor by Strieker, nor by Klein, nor by G']llacher, nor by 

 His. No more did Haeckel find any trace of it. I may then 

 venture to affirm, on the testimony of these various observers 

 and on the strength of my own researches, that the presence 

 of a cavity in the substance of the blastodisc is not a constant 

 fact; even more than this, no author has been able to ])rove its 

 existence in a living egg; it is therefore possible that in the 

 cases where it has been asserted to exist it is an artificial })roduct. 

 The absence of any regular disposition of the cleavage spheres 



