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MEMOIRS OF THE NEW YORK ACADEMY OP SCIENCES 



FIGUBE 19. 



Transverse section through "primitive streak" 

 region of a Noturus embryo of about the stage rep- 

 resented in Figure 22. 



appearance, a single continuous sheet, which later fused along the axial line with 

 the overlying cells; or was it differentiated from the overlying cell-layer, retain- 

 ing, however, its original continuity with the latter along the axial line ; or finally, 

 has the course of events been complicated by some process which we have left out 

 of consideration ? So far, my study of sections has not afforded a final answer to 



this question. In longitudinal sections, such 

 as those shown in figures 6 and 13, I can- 

 not find any evidence of a break in the con- 

 tinuity of this layer across the median line. 

 Since, however, the layer is not perfectly 

 distinct at all points, I cannot feel sure of 

 this fact. As stated above, transverse sec- 

 tions have so far failed to clear up the 

 matter. 



The relation of the gut epithelium to the notochord is also an interesting 

 problem. For some time after the chorda has separated from the neural axis above 

 and the mesoblastic plates on each side, there persists a continuity between it and 

 the gut-hypoblast. Does this continuity reveal the actual method of chorda forma- 

 tion in teleost ontogeny, or is the union a secondary one ? Also, what is the mean- 

 ing of the suggestive union occurring at certain points between the mesoblastic 

 plates and the hypoblast? (See figure 20.) Clearer light may be thrown upon 

 some of these points in the ensuing 

 discussion of the meaning of the 

 "prostoma." 



Before leaving this topic it is 

 well to emphasize, however, that a 

 part, though at present an uncertain 

 part, of the embryonic hypoblast is 

 derived from that collection of cells 

 which I have called the prostomal 

 thickening. Whether or not this 

 mode of origin is supplemented by 



differentiation from the overlying "secondary layer" of the germ-ring, I cannot 

 definitely say. 



There is certainly one region of the blastoderm where the hypoblast originates 

 quite independently of any connection with the prostomal invagination, namely, 

 the "non-embryonic " part of the germ-ring. Shortly before the blastopore closes, 



FIGURE 20. 



Section passing through posterior half of embryo of Noturus, 

 somewhat in front of Kupffer's Vesicle (cam. luc. ). The relations 

 of gut-hypoblast to mesoblast and chorda are suggestive. 



