(v- 

 Of THE 

 UNIVERSITY 



SUMNER: KUPFFER'S VESICLE 



57 



FIOUKE 16. 



SI 



tion is quite characteristic of the bony fishes, as witness the neural axis and auditory 

 and optic vesicles. The later appearance of the cavity of Kupffer's Vesicle is 

 strictly comparable with the delayed formation of the neural canal or the cavities of 

 the sensory vesicles. 



It may be objected that the sort of Kupffer's Vesicle, described for Ctenolabrus 

 and other pelagic eggs, in which a cellular floor is wanting, is not reducible to the 

 above type. But as already stated, I find in Cteno- 

 labrus a typical prostomal thickening, and I cannot 

 regard the fact that these cells do not completely 

 surround the cavity as of any importance to the 

 theory. I shall later give reasons for believing 

 that this condition is a retrograde one. 



Condition in Amia. Now is this condition in 

 the teleosts a unique one or do we find a counter- 

 part in any other group ? Dean has strongly main- 

 tained that the key to teleost development is to be 

 found among the ganoids, and from the generally 

 accepted phylogenetic relation of the two groups, 

 this proposition seems self-evident. 



It was DEAN'S endeavor (DEAN, '95, '96) to show 

 that the ganoids, in their mode of development held 

 a middle position between the elasmobranchs, on the 

 one hand, and the teleosts on the other. My own 

 observations as far as they go, sustain this view. As 

 regards the special subject of the present paper, 

 DEAN described in the eggs of Acipenser, Lepidosteus 

 and Amia, what he considered to be the homologue 

 of Kupffer's Vesicle in the teleosts. This was a 

 cavity beneath and slightly anterior to the dorsal 

 lip of the blastopore, bounded above and behind by 

 the cells of the latter, below by the yolk. DEAN 

 maintained that this cavity simply represented the 

 angle formed by the blastoderm margin as it was 

 mechanically turned in upon itself during its circumcrescence of the yolk. This 

 simple mechanical explanation I cannot accept for the teleosts because (among other 

 reasons) the vesicle in some fishes is not formed until the blastoderm has nearly or 

 quite finished its journey over the yolk (Muravna ? Perca) and thus the supposed 



Blastopore region of Amia cut in sagittal 

 plane (slightly schematized from camera 

 lucida drawing). Yc, yolk cells; Sy, yolk 

 syncytium; TV, " prostoraa " ; 81, superfi- 

 cial layer of epiblast. 



