376 LEFEVRE. [VoL. XIV. 
The Peribranchial Cavity. 
All investigators agree in deriving the peribranchial cavity 
from the inner vesicle of the bud-rudiment, but the manner in 
which it arises is not the same in all species of ascidians. 
In Perophora, Didemnum, and Amaroucium, according to 
Kowalewsky (12 and 13), two parallel longitudinal furrows 
appear on the outside of the inner vesicle, and by gradually 
deepening finally divide the latter completely into three por- 
tions. The two lateral divisions which are thus cut off grow 
up over the middle one, and fuse to form the median portion of 
the peribranchial cavity, which now surrounds the branchial 
sac dorsally and laterally. 
Seeliger (29) has described a different method of formation 
of the peribranchial cavity in Clavelina. According to him, the 
inner vesicle is not divided into three portions, but into two, 
one of which, the posterior, gives rise to the branchial sac and 
the gut, while the other forms the whole peribranchial cavity. 
These results were contradicted by Van Beneden and Julin 
(33), who maintained that in Clavelina the process is the same 
as that described by Kowalewsky. As Seeliger’s view was 
not founded on an investigation of an uninterrupted series of 
stages, and as there were wide gaps in his observations at 
veriods which are especially concerned in the formation of the 
peribranchial cavity, the supposition that his results are wrong 
is very probable. 
In Distaplia Salensky (27) has shown, and his results have 
been confirmed by Hjort and Frl. Bonnevie (10), that the inner 
vesicle gives rise to two lateral evaginations, which become 
completely constricted off as separate vesicles, the peribranchial 
sacs, and, gradually extending dorsally, fuse together on the 
median line. These sacs are not formed at the same time, but 
one is given off from the inner vesicle before the other, so that 
at a very early stage an asymmetry of the bud is produced. 
Della Valle (4) described a similar method of formation of 
the peribranchial cavity for Botryllus, but both Pizon (22) and 
Hjort (8) have conclusively proved his observations to be 
erroneous. According to these latter authors, both the lateral 
