244 • PROFESSOR HUXLEY ON THE ANATOMY 



tion to the nuclei of the division-masses — i. e., gave rise to tliem by immediate division. 

 The ovum is, in fact, more transparent than in other Rotifera ; and I have observed the 

 absence of the germinal vesicle." 



In a subsequent passage Dr. Leydig adverts to these observations as having inclined him 

 to alter his previously entertained opinions respecting the fate of the germinal vesicle. 



So far as the Vertehrata are concerned, such evidence as we possess as to the independent 

 origin of the embryo-cells appears to be altogether of the weakly negative sort. I do not 

 think it can be said that there is adequate foundation for the general assumption that the 

 contents of the germinal vesicle take no direct share in their production ; on the contrary, 

 as respects the Frog, I find definite evidence tending to a contrary conclusion. Prevost 

 and Dumas, and Von Bar, as is well known, proved the existence of a canal leading from 

 the centre of the dark part of the Prog's egg to a cavity wliich Von Bar considered to be 

 the seat of the germinal vesicle. Newport (Phil. Trans. 1851) described and figured this 

 canal and cavity, and showed that the germinal vesicle is, in the ovarian ovum, lodged in 

 the cavity. The vesicle is said to lie dense, white, and opake, and its interior to be full of 

 secondary cells*. Newport affirms that no trace of the vesicle is to be found in ova that 

 have left the ovary, but that an accumulation of white nucleated cells sometuues occupies 

 its place, in ova which are in the act of leaving the ovary. 



Ptemak (EntAvickelung der Wirbelthiere, 1855), apparently unacquainted with Newport's 

 observations, doubts whether the cavity down to which the canal leads, and which he 

 terms Von Bar's ' Kernhohle,' contains the germinal vesicle, though he inclines to the 

 opinion that it does. But it is a most important circumstance that he proves (I. c. p. 137) 

 that the division of this cavity accompanies each division of the yelk -mass, and that, 

 eventually, these cavities become what he terms the nuclei provided with nucleoli, which 

 occupy the centres of the division-masses of the yelk, and are the homologues of the 

 embryo-cells of Ascaris. If both Newport's and Remak's observations are correct, it 

 Avould seem impossible to deny that the embryo-cells of the Prog proceed from the con- 

 tents of the germinal vesicle. 



I think, then, that considering the only case in which the contents of the germinal 

 vesicle are not traceable, under circumstances in which it might be reasonably expected 

 that, if they really exist, they should be visible, is that observed by Kolliker ; while, on 

 the other hand, the equally definite observations of Nelson, MiiUer, Gegenbaur, and 

 myself (and the less distinct evidence of Newport, Bemak, and of Leydig) testify to 

 the origin of the blastoderm in one way or the other from the contents of the germinal 

 vesicle, in various members of no less than fourf out of the five primary divisions of the 

 animal kingdom ; the balance of the evidence is in favour of the conclusion that the em- 

 bryo-cells are the progeny of another cell, and that here, as elsewhere, extracellular cell- 

 development is a phenomenon of rare, if not of altogether questionable, occurrence. 



* Newport, it should be observed, used the term ' cell ' not very critically. But, ten years ago, cell-worship had 

 attained its culminating point. 



f Calenterata, Mol/itsca, Annulosa, Vertehrata. I may add, that the first appearance of the blastoderm on the 

 surface of the ovisac of Pyrosoma is so like that of the blastoderm in the ovum of any of the higher Articidafa, as 

 strongly to suggest a similarity of origin. 



