SUMNER: KUPFFER'S VESICLE 53 



in so far as they neglect the first steps in the formation of this interesting structure 

 and leave out of consideration certain phenomena of high theoretical importance. 



Relation of Kupffer's Vesicle to the "Prostomal Thickening." To return 

 to Salvelimisat a stage considerably later than that shown in figure 6, we have the 

 condition represented in figure 9, which, like the preceding figures, is drawn from a 

 median longitudinal section of the embryo. In the anterior portion of the section 

 there are three layers, not counting the overlying pavement layer, viz. : dorsally, 

 the neural axis, ventrally the single layer of gut-hypoblast, and between the two, 

 the disc-shaped cells on the notochord. The neural and chordal portions lose their 

 identity caudad in a homogeneous cell-mass. The ventral layer is continued back- 

 ward into one or both walls of Kupffer's Vesicle. The condition caudad to the 

 vesicle is less clear. It seems certain, however, that at this stage there is no distinct 

 ventral layer extending to the posterior margin. 



The position of this vescicle strongly suggests that it bears some relation to the 

 mass of cells which we have discussed under 

 the designation of " prostomal thickening." 

 Its walls are one or both continued cephalad 

 into the thin layer of gut hypoblast under- 

 lying the chorda. Moreover, this prostomal 

 cell-mass, unless it be represented by the 

 walls of the vesicle, has quite lost its iden- 



. Posterior end of sagittal section of brook trout 



tlty. Lhe most Conclusive evidence I shall embryo (camera lucida drawing). Kv, Knpffer's 



defer, however, until I have entered into a Veaicle : ^, neural axis; Ch, notochord; any, 



gut-hypoblast. 



brief historical discussion. Let us consider 



whether the structure herein termed " prostomal thickening " has been previously 



observed. 



Kowalewski's Account of Kupffer's Vesicle. M. VON KOWALEWSKI described 

 in Carassius a mass of cells, triangular in section, lying at the posterior margin of 

 the embryo and bounded by the pavement layer, the marginal wall and the periblast. 

 Except at this point, the pavement layer spanned a narrow space and ended freely on 

 the yolk as above represented. In his first paper (KOWALEWSKI '85) he considered 

 this cell-mass to be the rudiment of the whole entoderm, considering the cells of the 

 germ-ring to be purely mesodermal in their fate (he asserted this same view in his 

 next paper, '86, a). Later ('86, b) he modified his view to the effect that it repre- 

 sented only that part of the entoderm forming the walls of Kupffer's Vesicle. The 

 latter, according to KOWALEWSKI, was a part of the archenteron, and the cord of 

 cells which he figures (figure 10, B) as extending backward from below the vesicle 



