54 SCIENCE PROGRESS. 



primitive endodermal vesicle ; Oka (19), from the junction 

 between the pharynx and left peribranchial sac ; whereas 

 Hjort (9) maintains that the pericardial rudiment is not an 

 outgrowth of the inner layer at all. It is possible in this 

 case, as Salensky suggests, that Pizon has not really seen 

 the earliest stage of the pericardium. Salensky's obser- 

 vations on the buds of Distaplia (25), together with those of 

 Hjort on Botryllus, tend to show that the pericardium, in 

 some cases at any rate, may arise by the grouping of 

 mesoderm cells without any connection with the endoderm ; 

 though it is impossible to make a positive statement with 

 regard to either of the above instances. On the other 

 hand, the mesoblastic origin of the pericardium and heart 

 is perfectly clear in the buds of Pyrosoma, Salpa, and 

 Doliolum. In the two first cases the constituent cells are 

 derived directly from the pericardium of the parent ; in 

 Doliolum from the mesodermal element of the stolon, which 

 is nipped off from the mesoblastic bands of the larva. 



The musculature of Tunicate buds seems to be derived 

 from the mesoblast cells lying between the two primary 

 layers — if we except Uljanin's statements with regard to 

 Doliolum, which have been criticised above. Pizon's state- 

 ments concerning the existence of a special band of 

 ectoderm cells which give rise by proliferation to the 

 wandering mesoblast cells in Botryllus are entirely without 

 parallel, so that it is impossible to accept them until further 

 evidence is forthcoming. It must be remembered, more- 

 over, that this proliferation of the ectoderm has also been 

 observed by Oka, who states that the cells so derived 

 reunite beneath the dorsal tube to form the dorsal nerve- 

 cord. This assertion, although opposed by Pizon in an 

 appendix to his paper on Botryllus, appears to be still 

 worthy of consideration. 



The origin of the nervous system in the bud must next 

 engage our attention. Kowalevsky (16) and other earlier 

 investigators derived the nervous system from the roof 

 of the endodermal vesicle, as, for example, in Distaplia 

 stylifera ( = Didemnum styliferum of Kowalevsky), and 

 Amarcecimu. Seeliger, on the other hand, in the case of 



