302 Thos. H. Montgomery. 



for the first time appear distinct cell membranes, for heretofore the 

 cleavage cells lacked them ; these membranes arise by the cells coming 

 into mutual contact coincident with the shortening of their fibers. 

 Consequently, cell membranes are produced first at the ventral pole, 

 gradually they develop upon all cells of the germ disc as these cells 

 multiply and become more crowded. But the extraembryonic cells 

 remain membraneless until well past the gastrulation period, thus 

 retaining the character of early cleavage cells. 



The establishment of the germ disc seems due to two factors: (1) 

 Mainly to a more rapid multiplication of cells at the ventral pole, as 

 shown by their becoming progressively smaller there; and (2), to 

 less extent, by movement of cells toward that pole, shown by the cells 

 becoming less numerous on the dorsal hemisphere. 



2. The Earlier Part of the Gastrulation. 



The beginnings of the blastopore are found in eggs aged from 21 

 to 30 hours, when the blastoderm contains from 400 to 500 cells. On 

 unstained germ discs viewed in alcohol there appears an eccentric 

 whitish spot {Cum. A, PL II, Fig. 18), and this spot later includes 

 a small pit (Fig. 23) ; on stained preparations this shows darker than 

 the remainder of the germ disc (Figs. 15, 22). This spot is the 

 first region of cell immigration, whereas the remainder of the germ 

 disc is one-layered. It is always eccentric, and the margin of the 

 germ disc nearest to it will become the posterior or caudal region of 

 the embryo ; therefore, with its appearance there can be distinguished 

 for the first time anterior and posterior, as well as right and left. 

 For convenience this may be called the "anterior cumulus" {Cum. A 

 of the figures), since a second or "posterior cumulus" will later arise 

 behind it. 



A section of the earliest stage of the anterior cumulus of an egg 

 of 21 hours is shown in PI. II, Fig. 14; two cells have moved be- 

 neath the blastoderm in consequence of vertical mitosis. In an egg 

 of 24 hours about eight cells have invaginated; Fig. 15 is a ventral 

 view of the germ disc with about 417 cells (but its total number of 

 cells is somewhat greater, for a portion of the disc lies on the other 

 surface of the egg) ; Figs, 16 and 17 exhibit the anterior cumulus 



