No. I.] THE EGG OF THE TOAD-FISH. \\ 



of the ectoderm over the yolk. The ectoderm is sharply dif- 

 ferentiated from the ingrowing tongue of cells, especially at the 

 periphery. From this time the ring becomes less and less pro- 

 nounced. In surface views of the stage shown in PI. II, Fig. 8, 

 a little irregular thickening can be seen at the anterior pole, 

 and in sections a few scattered cells are found lying beneath 

 the thin, flattened ectoderm (PI. Ill, Fig. 6). In some sections 

 not even this much of the thickening remains, as the cells occur 

 in patches. In the lateral region the reduction is not yet car- 

 ried so far (PL III, Fig. 7). Sections through the rim of the 

 stage shown in PI. II, Fig. 9, have no thickening even in the 

 lateral region, while the tongue of cells at the embryonic pole 

 is steadily lengthening. 



In Ctenolabrus Dr. Whitman finds that there is "a plain rolling 

 under or involution as an initiatory step in the formation of the 

 ring," but believes that the process is more correctly described 

 as "an ingrowth due both to a rapid multiplication of the cells 

 and also to the centrifugal expansion of the ectoderm." At 

 the posterior margin "the inroUing portion presents a strongly 

 voluted outline, while- at the anterior border it is much more 

 feebly expressed." ^ In Batrachus, around most of the margin 

 there is found simply "the initiatory step," and even that 

 lacks the voluted outline, except at the embryonic pole. The 

 loose cells budded off from this small peripheral thickening 

 represent, I believe, a true germ-ring. In the Sea Bass, Pro- 

 fessor Wilson finds, at the embryonic pole, an apparent invagi- 

 nation caused by a centripetal growth of cells, and forming a 

 Randwulst from which cells are proliferated centripetally to form 

 a germ-ring. " Round the rest of the edge the ingrowth is like- 

 wise, at least in most places, preceded by the formation of a 

 Randwulst, which, however, is inconspicuous." 



From the stage given in PL II, Fig. 9, down to the closure 

 of the blastopore at a distance behind the embryo, there is an 

 apparent marginal thickening visible even in the living egg (PL 

 II, Figs. 3-5). In specimens killed in Perenyi's fluid, a distinct 



1 Alexander Agassiz and C. O. Whitman, " On the Development of Some 

 Pelagic Fish Eggs." Preliminary notice. Proceedings of the American Academy 

 of Arts and Sciences, vol. xx. 



