SUMMER : KUPFFER'S VESICLE 73 



VI. FURTHER MORPHOLOGICAL CONSIDERATIONS. 



The generally accepted view that Kupffer's Vesicle represents a certain part of the 

 archenteron seems to me to be true beyond doubt. How much of the gastrula cav- 

 ity is represented by this structure is less obvious. It has been variously inter- 

 preted as the allantois, as part or whole of the post-anal-gut, or as the latter plus the 

 neurenteric canal. The case of Murasna f seems to show that the neurenteric canal 

 is not included in the vesicle proper, but that the latter forms the dilated inner end 

 of the imagination which gives rise to both. The neurenteric canal, in most teleosts, 

 is represented by a solid ingrowth as maintained by KOWALEWSKI. At the inner 

 end of this, the cavity of the vesicle forms secondarily. 



In many fishes, the post-anal portion of the gut FIGURE 32. 



(Kupffer's Vesicle) possesses from the first, as we 

 have seen, a ventral as well as a dorsal wall. In 

 the more anterior portion, or gut proper, the lumen 

 is formed by a real or virtual upfolding of a hori- 

 zontal sheet of cells, the axial portion of which rep- 

 resents the dorsal wall of the gut, the lateral por- 



Sagittal section showing late Kupffer's 



tions representing the ventral. We should thus Vesicle in 



expect the gut-hypoblast to be continued into the 



dorsal, rather than the ventral wall of Kupffer's Vesicle. This relation is impossible 

 to determine at an early period, but figure 32 representing a late stage of the vesicle 

 in the trout embryo, exhibits the expected condition. 



The structure of the fully formed Kupffer's Vesicle and adjacent parts have been 

 so often carefully described that they need not be discussed here except in connec- 

 tion, with certain differences, already mentioned, in the form which it assumes in 

 various types of fishes. Two different types of Kupffer's Vesicle have been de- 

 scribed above, one with no cellular floor, the other from the first bounded on all 

 sides by cells. That these two types exist there can no longer be any doubt. Ac- 

 cording to the view maintained in this paper as to the significance of this structure, 

 it is obvious that I must regard the second type as the more primitive. The forms 

 in which the cellular lower wall has been described as wanting are, as far as I know, 

 all rapidly developing pelagic eggs, in which many developmental processes are 

 modified by abridgement. On the other hand the more slowly developing cat-fish, 

 trout and salmon, in which there is reason to believe that the type of development 

 is less modified, display a vesicle with a complete cellular boundary. An exception 



