370 LEFEVRE. [VoL. XIV. 
koff nor Krohn (15), who confirmed these observations on the 
Botryllus bud, saw at all clearly the details of the development. 
This was left for Kowalewsky, who described many of the 
internal processes occurring in the bud development of Pevo- 
phora listert. He showed that in Perophora, also, the young 
bud is composed of two vesicles, the outer one being derived 
from the ectoderm and the inner from the partition or cloison 
of the stolon. According to Kowalewsky’s account, the endo- 
dermal or inner vesicle becomes divided completely into three 
portions, the two lateral fusing dorsally and forming the peri- 
branchial cavity, and the median giving rise to the branchial 
sac. I shall try to show below that in Perophora viridis, at all 
events, the peribranchial cavity is formed by quite a different 
process. 
The origin of the bud as a double-walled vesicle has been 
verified by all subsequent investigators, and thoroughly estab- 
lished as a type of development common to all ascidians whose 
budding has been studied. 
The outer wall of the vesicle is directly derived from the 
ectoderm of the parent animal, and becomes the ectoderm 
of the bud. According to the majority of investigators, this 
outer layer takes no active part in the further development, 
but Salensky (27) and Oka (20), as will be pointed out below, 
maintain that the ectoderm is concerned in the formation of 
the nervous system. 
As to the derivation of the inner wall of the vesicle, the case 
is not so simple, for in different ascidians this layer may arise 
from entirely different parts of the parent, coming in some 
forms from an endodermal, in others from an ectodermal struc- 
ture. In Perophora and Clavelina it is derived from the cloison 
or septum of the stolon, which in the latter, and presumably in 
the former, is of endodermal origin; in Didemnum and Dis- 
taplia from the wall of the gut, and in the Polyclinidae from 
the endodermal wall of the post-abdomen. 
In all the above-mentioned species, then, the inner vesicle of 
the bud-rudiment is derived from an endodermal structure. In 
Botryllus, however, this inner vesicle is formed directly as an 
evagination of the outer wall of the peribranchial sac, whose 
