394 LEFEVRE. [VoL. XIV. 
but the posterior is higher than the anterior end. This 
inclination is seen in Pl. XXIX, Figs. 4, 5, in which the peri- 
cardium is indicated at fc. In the latter figure, which shows 
about its final position, the pericardium is seen to extend from 
a point at a level nearly as high as the upper end of the 
stomach straight down to the stalk which connects the bud with 
the stolon. 
The Dorsal Tube and Ganglion. 
Of all the organs of the ascidian bud, that which has given 
rise to the greatest amount of discussion is the nervous system. 
Its origin and development have been matters of much dispute, 
and so wide is the difference of opinion concerning points of 
fundamental importance that there is little hope at present of 
harmonizing the conflicting statements of various authors. 
A close relation between the dorsal tube and ganglion has 
been affirmed by many who hold to a common origin of the two, 
but is strenuously denied by others, who assert that the dorsal 
tube arises independently and has nothing whatever to do with 
the nervous system. Different authors have ascribed to these 
structures an ectodermal, a mesodermal, and an endodermal 
origin, and have thereby exhausted the entire series of pos- 
sibilities. 
Kowalewsky (12), for the buds of Perophora, was the first to 
describe an endodermal origin of the nervous system. Accord- 
ing to him, the dorsal wall of the branchial sac evaginates to 
form a tube, which retains its connection with the branchial 
cavity, and which he calls the “Nervenrohr.” In his later work 
on the budding of ascidians (13) he describes the rudiment of 
the nervous system in Amaroucium and Didemnum as “ ein sehr 
langes, am vordern Ende ziemlich breites Rohr, dessen Lumen 
mit der Héhle des Kiemensackes zu communiciren scheint ”’ 
(/.c., p. 465). He did not follow the development of this tube, 
which he held to be derived from the endodermal vesicle, and 
was ignorant of its relation to the nervous system of the adult 
animal. It is probable, however, that he saw the ganglion in 
Amaroucium, at least, but failed to recognize it, for he says 
(/.c., p. 465), ‘‘ Bemerkenswerth ist noch, dass iber dem Nerven- 
