G2 



MEMOIRS OF THE NEW YORK ACADEMY OF SCIENCES 



IV. SIGNIFICANCE OF THE PROSTOMAL INV AGINATION. 



I now purpose to offer an interpretation of the foregoing phenomena and to 

 point out their relation to phases in the development of other vertebrates. 



My theory in large degree reverts to that proposed by KUPFFER in 1884. (Prac- 

 tically formulated in 1879 see KUPFFER, '79.) KUPFFER'S observations, already 

 discussed, led him to a novel view of the development of the fish embryo. It 

 was his endeavor to make a complete comparison between the conditions found 

 in the Teleostei and that found in the Amniota. KUPFFER was the first to point 

 out the dorsal invagination in the reptilian blastoderm, and he considered that 

 in this he had found the homologue of the gastrula mouth or blastopore of 



FIOUBE 22. 



FIGURE 23. 



Pr- 



Early blastoderm of Noturus. Gr, germ- 

 ring ; Pr, "prostoma." 



Early embryo of Notaries, before appearance of 

 caudal knob. The bifid caudal end of the embryo 

 si seen to embrace a thin cellular sheet as in case 

 of Murxna ? (see figure 27). 



Amphioxus. The yolk-filled aperture of the blastoderm margin had nothing to 

 do with blastopore proper. For this aperture he invented the name of " BLAS- 

 TOTREMA." He considered the process by which the blastoderm covered the yolk 

 as in no sense a process of gastrulation but as a process of blastosphere formation, 

 thinking that the completed blastosphere would surround the yolk on all sides. 

 KUPFFER sought for and found a homologue of the reptilian prostoma in teleosts. 

 This invagination, he says, at first + shaped in . surface view, greatly elongates 

 in a forward direction, resulting in the formation of a longitudinal groove which he 

 calls the " primitive groove," extending along the axis of the embryo. He dif- 

 fers in his interpretation of the primitive groove and streak from BALFOUR, who, 

 as well known, considered these structures to be homologous with the seam along 

 with the blastoderm edges united behind the embryo in the elasmobranch (figure 

 29, A, Bl). Thus, according to BALFOUR ('78 and '81) the primitive streak is posterior 



