SUMNER : KUPFFER'S VESICLE 



51 



Figure 5 represents a sagittal section through an early blastoderm of the brook 

 trout (Salvelinus fontinalis), 1 absolute age unknown. One of the most obvious and 

 striking facts is the condition of the pavement layer at the posterior (embryonic) 

 margin. Its cells, instead of being thin and flattened as elsewhere, are here elon- 

 gated in a vertical direction. The surface of the blastoderm at this point is notice- 



FlOURE 5. 



Pr, 



^M^^v^l^/^^ 



?:>: i&Jsffi*'s'$< & *' - : J>^">i '^'M'^W 

 '.'ri^y*'. ^.y.'a-if^-^-'w-.r >; '/ J >^ : -;. - .< /"..-:^s^.i{, 



W^iiK fe;: ^" 



After camera lucida drawing. Pr, thickening of pavement layer at posterior border. 



ably indented, and the arrangement of the cells with reference to the indentation is 

 such as to strongly suggest an invagination occurring here. The line separating the 

 " Prostomal Thickening," as I shall henceforth call this proliferation of the super- 

 ficial layer, and the underlying cells of the blastoderm is not at all clear in this 



FlGtTEE 6. 



Pr 



Sagittal section (after camera Incida drawing) showing condition in Snleelinus blastoderm considerably 



more advanced than that shown in Figure 5. 



section. The deeper stain of the superficial cells nearly disappears as we pass inward 

 from the surface. 



At a somewhat later period (a day or less older) the germ-ring is well established. 

 It will now be seen (figure 6) that the " prostomal" mass of cells is continued for- 

 ward into a thin layer underlying the cells of the germ-ring proper. 



1 It cannot be objected that the difference may be due to my having studied a different species of trout, since, as we 

 shall see, the condition described seems to be a universal one among the Tdeontei. 



