382 LEFEVRE. [Vou. XIV. 
of the branchial sac. Fig. 17, line 4, only includes the anterior 
extension of the right sac (7.a.ex.), for, as just stated above, the 
pouch on the opposite side lags behind in its growth. Fig. 18, 
line c, represents a section taken just in front of the anterior 
face of the cloaca, and shows both peribranchial sacs at the 
level where they are continued forward into their anterior 
extensions. In Fig. 19, line a, the section passes through the 
anterior portion of the cloaca (c/.), which is seen to connect the 
lateral sacs; the constriction, which will ultimately completely 
separate the saddle-shaped bag from the median vesicle, has 
proceeded in this region to a considerable extent, and has 
greatly narrowed the opening between the peribranchial and 
branchial cavities. In a section further back (Fig. 20, line e), 
the folds which are forming the peribranchial sacs are much 
less deep and wider apart; the connection between the left sac 
and the stolonic partition is present in this region (s¢.c.). Finally, 
Fig. 21, line f, represents a section beyond the peribranchial 
sacs, the posterior pouches of which at this stage have not yet 
begun to grow out; the section passes behind the connection 
with the stolonic partition, but through the intestine (zwz.), 
which is seen on the left side. 
The further development of the peribranchial cavity from 
this stage on merely consists in the completion of the constric- 
tion, whereby the saddle-shaped bag is completely cut off from 
the branchial sac, and in the extension of the anterior and 
posterior pouches of the peribranchial sacs, which finally sur- 
round the whole pharynx laterally. These relations are illus- 
trated by the series of sections (Figs. 22-24), which are taken 
from the same bud. Fig. 22 is a section through the anterior 
end of the bud, and shows the lateral extensions of the peri- 
branchial sacs (7.a.ex. and /.a.er.) surrounding the pharynx 
(4r.s.). The peribranchial cavity is now entirely cut off, and its 
lateral portions are united in the middle region of the bud by 
the dorsal connecting piece or cloaca; this condition appears in 
Fig. 23. Beyond the cloaca the posterior pouches of the lateral 
sacs, which, like the anterior pouches, are not united dorsally, 
are seen in Fig. 24 (~p.er. and /..ex.); the connection of the 
left sac with the stolonic partition in this figure has already 
