ASCIDIACEA FORMATION OF ORGANS IN THE BUD. 



467 



The observations of VAN BENEDEN and JULIN, made on the buds 

 of Clavelina, on the other hand, lead them to assume an ectodermal 

 origin of the neural tube. In early stages, this tube is closely 

 applied to the ectoderm and, in its posterior strand-like continuation, 

 the boundary between the ectoderm and the nervous system is quite 



FIG. 238. Diagrammatic cross-sections through the posterior part of a bud of Clavelina 

 (after VAN BENEDEN and JULIN). ep, epicardium ; g, genital vesicle ; h, heart ; i, 

 intestine; ,n , stomach; n, visceral nerve-strand; oe, oesophagus; pc, pericardia! 

 vesicle ; ph, posterior wall of the pharynx with the aperture of the oesophagus and 

 the paired apertures of the epicardial tube ; st, stolonic septum. 



obliterated (Fig. 238 D-F, n). [SEELIGER (No. XXXIV.) has rein- 

 vestigated the bud of Clavelina, and finds that the nervous system 

 is unmistakably entoderrnal in origin, and in this conclusion he is 

 supported by BITTER'S (No. XXVIII.) investigation on Perophora, 

 one of the Clavelinidae.] 



