SUMNER: KUPFFER'S VESICLE 61 



that part of the blastoderm forming its now posterior margin (in Noturus and 

 Murxnaf) is seen to be differentiated into all three germ-layers (see figure 14). 

 VIRCHOW ('95) describes this condition in the trout and I ('99, a) have already re- 

 corded it for Noturus. 



Historical. With the exception of the earlier investigators, who derived the 

 entoderm from the periblast, nearly all writers on fish embryology have thought 

 this germ-layer to arise as a differentiation from the inflected layer of the germ- 

 ring. The cells of the latter were held to separate sooner or later into two layers, 

 the lower of which was the entoderm, the upper the mesoderm. The only writers, 

 as far as I know, who have maintained the existence of a distinct entoderm rudi- 

 ment are KOWALEWSKI (see above), BERENT ('96), and REINHARD ('98). 



BERENT maintains that the entoderm arises as a separate rudiment and has 

 figured it with approximate correctness in one stage (figure 21, B). But his inter- 

 pretation of the process is certainly wrong. The accompanying figure 21, A from 



FIGURE 21. 



After Berent illustrating his view of entoderm formation. 



BERENT is probably based upon an observation, though an inaccurate one, of the 

 earlier appearance of this same group of cells. He holds the condition described 

 by H. V. WILSON ('91) for the sea-bass (i. e., a uniform differentiation of the under 

 surface of the whole embryonic germ-ring) to be the more primitive, and considers 

 the condition he finds in the trout to be a derived one. My objections to BERENT'S 

 conclusions are two : first, that the rapidly developing pelagic egg of the sea-bass 

 would be far more apt to exhibit a precocious development of any part than the 

 slowly developing egg of the trout, and second, that the account offered by WILSON is 

 undoubtedly incomplete. While I have never studied the egg of the sea-bass, I 

 have carefully sectioned the appropriate stages of the quite similar egg of Ctenola- 

 brus, and find that here, as in the eggs of so many widely different fishes, the gut- 

 hypoblast is plainly formed in connection with a prostomal ingrowth. Unlike 

 KOWALEWSKI, BERENT has missed the true meaning of the process. REINHARD re- 

 gards the entire hypoblast as derived by proliferation from the walls of Kupffer's 

 Vesicle. The cells of the latter arise, he claims, from the periblast. 



