174 tel Pete Te. [VoL. XII. 
c. The Pericardium and Heart. 
The origin of the common Az/age of these two structures 
has been the subject of almost as much diversity of statement 
for different groups of Tunicates as has been the origin of the 
nerve ganglion. It is, therefore, particularly satisfactory to 
find that in this species its method of origin is so clear as 
to leave no doubt about what it is. 
At the time of writing my preliminary communication I had 
not succeeded in finding the earliest stage in the development 
of the structure ; further search, however, since then has borne 
the desired fruit. 
It arises from the postero-ventral wall of the inner vesicle, 
but does not become fully severed until the ventral folds which 
play the major part in separating the branchial from the peri- 
branchial sacs have extended nearly back to the intestinal 
Anlage. These folds, consequently, enable us to locate more 
precisely the point of its origin. 
Figs. 39-41, Pl. XV, are from a series of transverse sections 
of a bud in a stage of development considerably younger than 
that shown in Fig. 18, Pl. XIII. Fig. 39 represents the tenth 
section in front of the one from which Fig. 40, passing through 
the pericardial An/age, pc.a., is drawn. The ventrally directed 
pouch-like fold, 70.s"., seen in Fig. 39, is a portion of the 
right peribranchial sac, and it is at the posterior extremity of 
this that the pericardial Ax/age originates. As shown most 
clearly in Fig. 41, it is an evagination from the inner vesicle, 
but, as just stated, is at the point reached at this stage by the 
posterior extremity of the right peribranchial sac. vf, in 
Figs. 40 and 41, indicates the right ventral partitioning fold. 
The Anlage of the pericardium appears to become separated 
from the vesicle at its anterior end first, then later at its 
posterior end. A later stage, slightly after its complete separ- 
ation, but while it is still a simple small vesicle, is shown in 
Fig. 42, pc.v. 
According to Oka (92), p. 535, the pericardium in Botryllus 
arises as “eine solide Wucherung des inneren Blattes der 


