392 LEFEVRE. [VoL. XIV. 
origin of the peribranchial rudiment cannot, however, be unhesi- 
tatingly accepted, since his figures do not satisfactorily establish 
the correctness of his description, while the supposition that 
he has not followed the development with sufficient care is very 
strong. Salensky (27, p. 527) calls attention to the fact that 
the little circle of epithelial cells which Pizon marks with the 
letters Per in Pl. I, Fig. 7, “ wohl auch einen Querschnitt der 
unteren Wand des Kleimendarmes darstellen kann,’ and that 
it is not at all proved that it is the same structure as the 
pericardium, figured in later stages. 
The first appearance of the pericardial rudiment observed by 
Hjort (8) in Botryllus was a small clump of cells lying against 
the ventral wall of the inner vesicle in the posterior part of the 
bud to the right of the middle line. As to the derivation of 
these cells, Hjort was unable to say whether they were meso- 
dermal cells or cells which had wandered out from the endo- 
derm, but he distinctly states that an evagination of the inner 
vesicle does not occur at this point. 
In the buds of Distaplia, Salensky (27) observed a similar 
collection of cells lying against the lower wall of the branchial 
sac and surrounded by mesodermal cells. He maintains that 
there is no ground for attributing an endodermal origin to the 
rudiment, which is from the beginning sharply marked off from 
the wall of the branchial sac, and he therefore concludes that 
the pericardium is derived from the mesoderm. 
The result to which my observations on the bud development 
of Perophora viridis have led me, in regard to the origin of the 
pericardial rudiment, is in accord with that of Salensky. 
At about the stage represented in Pl. XXX, Fig. 9, a very 
loose patch of cells (fc.v.) is found applied to the outer surface 
of the inner vesicle high up on the right side in the posterior 
end of the bud. Before this time many isolated cells are seen 
adhering to the wall of the vesicle at numerous points (Figs. 7, 
8), but when the difference in thickness between the right side 
and the rest of the vesicle is just becoming apparent a marked 
tendency in the scattered cells to accumulate in one spot is 
noticed. At first there is but a single layer of cells joined 
loosely together end to end and forming a somewhat elongated 
