50 



MEMOIRS OF THE NEW YORK ACADEMY OF SCIENCES 



As to the mechanics of the process, GOTTE maintained that the same peripherad 

 movement of the cells which produced the " Randwulst " produced an involution of 

 the margin to form the " Secundiire Keimschicht," and led to the growth of the lat- 

 ter centrad. 



Although other writers have held, and probably with truth, that at least in 

 some cases, the secondary layer is formed wholly or in part by ddamination from the 

 germ-wall, nearly all ernbryologists are agreed that the end result is as shown in 

 figure 4. A circular sheet of cells, inflected at its margin, and covered by a pave- 

 ment layer which does not share in this inflection such is their teleost gastrula. 

 According to this view the germ-ring or inflected layer represents the entire hypo- 

 blast and mesoblast of the embryo. 



A New Factor. One object of the present paper is to show that the above 

 conception of gastrulation in the teleost, although in large degree true, fails to 



FIGURE 4. 



recognize a highly important factor. The fish gastrula presents a far more complex 

 structure than the scheme of Gotte contemplated. 



More than a year ago, in a paper before the American Morphological Society 

 (December 28, 1898) the writer described for the cat-fish blastoderm a pronounced 

 thickening of the pavement layer (" Deckschicht ") on the embryonic (posterior) 

 margin of the blastoderm. Although stating that this structure was of constant 

 occurrence, I was at that time unable to offer an opinion as to its significance. I 

 have later devoted a great deal of attention to this question and have discovered 

 this same problematic cell-mass in a considerable number of fishes belonging to 

 widely different families. In all these cases, the origin and fate of this cell-mass 

 appears to be the same. It is perhaps most readily observed in the trout ; but 

 although this fish has long been a classical object for embryological study, the 

 appearances which I am about to describe have been noticed by only one previous 

 writer (Berent, '96). 1 



1 BERKNT'S observations were quite incomplete and his interpretations entirely incorrect. Even GREGORY ('99) 

 the latest of those who have discussed the early development^ of the trout, has missed the point as completely as any of 

 his predecessors. 



