374 LEFEVRE. [Vou. XIV. 
It now represents the characteristic double vesicle of the 
young ascidian bud; the outer or ectodermal vesicle is directly 
derived from the ectoderm of the stolon, and the inner or ento- 
dermal vesicle, which has become a closed sac, arises by evagi- 
nation of the thickened dilated portion of the stolonic partition. 
The connection between the partition and the inner vesicle is 
retained for a long time, and the cavity of the latter is to be 
regarded as being in communication with the potential cavity 
of the partition. 
A transverse section at this stage (Pl. XXX, Fig. 8) shows 
the still somewhat cuboidal cells of the ectoderm of the bud- 
rudiment, and the endodermal vesicle (¢z.v.), with its much 
thickened walls lying inside. Active cell multiplication has 
been going on in the latter, which are of nearly uniform thick- 
ness, except below, where they pass off gradually into the thin 
walls of the partition. The cells of the blood are found scat- 
tered about between the two vesicles, and it is to be especially 
noted that many are lying close against the inner surface of the 
ectoderm and outer surface of the endoderm at numerous 
points (w2.c.). 
The bud-rudiment does not long remain spherical, but soon 
becomes elongated by a growth towards the apex of the stolon. 
It now assumes an oval shape, and one end lies free over the 
surface of the stolon, beyond the portion which is directly con- 
nected with the latter (Pl. XXIX, Fig. 3). It is the free end 
which is later to be distinguished as the anterior portion of the 
bud, while by this process of elongation anteriorly the connec- 
tion with the stolon comes to be left behind in the posterior 
region. The side next the stolonic wall becomes the ventral 
surface of the bud, and that opposite it the dorsal. 
The protuberances of the stolonic ectoderm, which Seeliger 
(29) describes as occurring at the base of the bud-rudiment in 
Clavelina, are not present in Perophora; they are merely simple 
ectodermal sacs filled with blood cells, and do not contain an 
evagination of the partition. Seeliger calls them ‘ Nahrkam- 
mern,” and regards them as reservoirs of food material for the 
developing buds. 
Before going on to describe the origin of the various internal 
