is merely a part of the stolonic partition-wall and is not 

 concerned in the formation of the heart. According to the 

 Belgian authors, who described in detail the development of 

 the pericardi;ini in the buds of Clavelina, the pericardium and 

 epicardium at first form a common cavity with the inner ves- 

 icle. A separation takes place later in such a way that the 

 epicardiiim remains in communication '.vith the inner vesicle, 

 while the pericardium becomes entirely cut off from the lat- 

 ter, but retains its connection with the stolonic partition. 

 Van Beneden and Julin maintain that the union of the epicar- 

 dium with the branchial sac is never lost in the bud-develop- 

 ment of Cla velina, and therefore do not agree with Seeliger's 

 statement that the diverticulum, which he observed and er- 

 roneously regarded as the pericardium, beco?'ies separated from 

 the branchial sac. Seeliger's description of the early con- 

 striction of the inner vesicle from the stolonic partition is 

 not confirmed by the Belgians, who showed that the pericar- 

 diiim, originally a part of the inner vesicle preserves its 

 connection with the partition wall, as explained above. 



A somewhat similar origin of the pericardium to- 

 gether v/ith the epicardium from the inner vesicle is stated 

 by Pizon (22) to occur in the P olycl inidae, for example in 

 Circinalium and Amarou oium. 



Our knowledge of the derivation of this structure 

 in the buds of Botryllus is very much less certain. Pizon 

 (22) declares that the pericardium arises as a little diver- 

 ticulum from the lov/er wall of the inner vesicle, which be- 



