>. 
Novi) (bCDDING IN GOODSIRIA’ AND PEROPHORA. 167 
represented by Fig. 16, while neither is yet indicated in the 
bud with the peribranchial evaginations, also suggests that the 
former is older than the latter. But the discrepancy between 
the two conditions may be explained in another way; it may 
be due to individual variation ; in any case, however, it cannot 
be a matter of fundamental importance. 
The initial steps in the formation of the peribranchial sacs 
accomplished, the remainder of their development is followed 
without difficulty. The sickle-shaped partitioning folds, .f, 
Fig. 17, extend their arms, one on the dorsal, the other on the 
ventral side of the inner vesicle, farther and farther backward. 
But of the two arms of each fold, the ventral one takes by far 
the more important part in effecting the ultimate complete 
separation of the peribranchial from the branchial sacs. 
The process is clearly illustrated by Figs. 16-19, Pl. XIII, 
representing dorsal views of whole buds examined as transpar- 
ent objects; and by Figs. 22-24, Pl. XIII, and Figs. 26-20, PI. 
XIV, inclusive. The series of transverse sections represented 
by Figs. 22-24 are from a bud in a stage of development 
slightly more advanced than that shown in Fig. 17, Pl. XIII. 
Fig. 22 is the most anterior of the three sections drawn, and, 
as will be seen, passes through the point at which the bran- 
chial siphon, 67.52%., is being formed. Fig. 23 is fourteen 
sections farther back. It shows both the dorsal and ventral 
arms of the sickle-shaped folds, @.f. and v.f., those of the right 
side (left side of the figure) very nearly meeting each other, 
and so making the separation of the right branchial sac at this 
point nearly complete. Fig. 24 represents a section farther 
back, and shows at this position merely a trace of the folds, 
the three sacs of the anterior region being here merged into 
one common cavity representing the unmodified remnant of the 
primitive inner vesicle. Figs. 26—29, Pl. XIV, represent similar 
sections of a still older bud. That shown by Fig. 26 is most 
anterior, and passes through the opening of the hypophyseal 
duct into the branchial sac, Zy.m., and hence is slightly farther 
back than the section shown in Fig. 22 of the preceding series. 
Fig. 28 represents a section sixteen sections farther back. It 
is seen that the dorsal folds take no part in the formation of 
