No. 3-] BUDDING IN PEROPHORA. 381 
as well as a short region at the posterior end of the vesicle. 
And, further, the whole peribranchial cavity is not constricted 
off at the same time, but, as stated above, the furrows begin 
anteriorly and extend gradually back, so that at any given stage 
the opening of the median vesicle into the peribranchial cavity 
is much wider in a posterior section than in one further 
forward. 
When the right and left peribranchial sacs are being formed, 
as just described, a broad pouch or diverticulum grows out from 
the anterior margin of each, and by degrees spreads over the 
undivided portion of the original inner vesicle. These pouches 
are direct continuations of the lateral cavities, and later com- 
pletely cover the sides of the anterior region of the peribranchial 
sac, but they never fuse dorsally. 
Similar extensions are carried out from the posterior margin 
of the lateral cavities, and though not prominent at first, still, 
as the bud gets older and increases in length, they attain a con- 
siderable size and surround a part of the digestive tract. 
The peribranchial cavity now consists of two deep lateral 
sacs, surrounding the spacious branchial sac, and connected 
dorsally by a median space, the cloacal cavity or atrium. The 
lateral sacs are unsymmetrical, however, until quite a late 
stage, for the anterzor pouch of the rzght peribranchial sac 
grows more rapidly and extends further forward than the simi- 
lar pouch on the left side, while the posterior pouch of the left 
side extends further back than the corresponding one on the 
right. Eventually the two sacs become symmetrical. 
The formation of the peribranchial cavity is easily understood 
from the series of sections represented in Pl. XX XI, Figs.16-21; 
these will be rendered more intelligible by a comparison with 
Pl. XXIX, Fig. 4, which is drawn from a total preparation of 
a bud at the same stage of development. The sections are 
taken respectively at the levels indicated by the parallel lines, 
a Oru cuanoane Ok rie. As) In. Ply XX XI, Fic. 16, line ot 
Fig. 4, the most anterior one of the series, the branchial sac 
(dr.s.) is seen by itself, for the extensions of the peribranchial 
sacs have not reached far enough forward to appear in the 
section; the hypophyseal tube (d@.7.) is shown on the dorsal side 
