168 RITTER. [Vox. XII. 
the peribranchial sacs in this region. Eight sections farther 
back, Fig. 29, the ventral folds no longer appear, and we have 
again an unpartitioned portion of the primitive vesicle. 
From the figures of the whole buds, and also from those of 
the series of sections, it will be seen that the folds do not 
extend back parallel to each other, neither do they occupy a 
vertical position. They converge both posteriorly and ven- 
trally, so that the middle or branchial sac is made cone-shaped, 
the apex of the cone being directed backwards. This becomes 
still more distinct at a later stage in development (see Figs. 
18 and 19, Pl. XIII). An inclination toward the left side of 
the posterior extremity of the cone is now noticeable, and this 
becomes more pronounced at a later time (compare Figures 
mentioned). In some buds it is much more prominent than 
in others. 
We may now pass to the practically completed condition of 
the branchial apparatus. A dorsal view of an entire bud in 
such a stage is shown in Fig. 19. The only changes that will 
take place between this and the fully adult condition will be 
some unimportant ones, so far as our present purpose is con- 
cerned, in the relations of some of the parts to one another, 
and a great increase in size of all the parts. From the prece- 
ding stage the most important changes have perhaps been those 
taking place in the posterior region of the animal. 
Recurrence to Figs. 23 and 24, Pl. XIII, will recall the fact 
that at this stage the three sacs opened widely into a common 
chamber in the posterior part of the bud. In the stage we are 
now considering such is no longer the case. The two ventral 
folds by growing up to and fusing with the dorsal wall of the 
primitive vesicle for some distance, and then by fusing with each 
other but xot with the dorsal wall, for the rest of the way back 
to their extremities have effected the complete separation of 
the branchial sac from the peribranchial sacs. These relations 
will be easily understood by comparing Fig. 24 of the preceding 
stage with Fig. 28 of the present one, and then Figs. 30 and 
31, Pl. XIV, with Fig. 28. Figs. 30 and 31 are from a bud 
considerably more advanced than the one represented by 
Figs. 26-29. Fig. 31 shows at drs. the posterior tip of the 
. 
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