STUDIES ON THE PROTOOHOEDATA . 313 



definite cerebral ganglion on the left side (right of the figure) 

 of the cerebral vesicle just above the hypophysis tube. The 

 continuity of the lumen of the latter can be traced in the 

 clearest manner from the anterior branchial opening to the 

 section under consideration. The extreme posterior termina- 

 tion of the hypophysis was rather difficult to make out at this 

 stage, and between the sections drawn in figs. 44 and 45 there 

 seemed to be an interruption in the continuity of the lumen. 

 This probably is an indication of the eventual closing up of the 

 hypophysis posteriorly. 



As to the actual origin of the cells which compose the 

 permanent cerebral ganglion, it is undoubtedly correct to say 

 that they proceed, together with the hypophysis, from the cells 

 which form the left dorso-lateral portion of the wall of the 

 cerebral vesicle (cf. figs. 34 — 37). This becomes especially 

 obvious by the study of such a series of sections as that from 

 which figs. 43 — 46 were taken. In the hinder region of the 

 cerebral vesicle the boundary line between the hypophysis and 

 the developing ganglion was by no means so distinct as it is in 

 fig. 44. 



It is clear from the above description that in Clavelina the 

 formation of the permanent ganglion commences at a relatively 

 much earlier stage than it does in Ciona — in fact, before the 

 atrophy of the cerebral vesicle; and we see, further, that it is 

 from the beginning a solid structure. Van Beneden and Julin 

 appear to have mistaken the developing hypophysial tube for the 

 developing ganglion. Some of their figures coincide fairly 

 closely with some of mine, but they have interpreted them 

 totally differently, and, it must be added, to a large extent 

 erroneously. They agree with Seeliger in saying that the hypo- 

 physis, including for their part emphatically its glandular por- 

 tion, is entirely derived from an evagination from the wall of the 

 branchial sac, to which they give the name of the '^ caecum 

 hypophysaire," which applies itself against the cerebral 

 vesicle, but never communicates with it. My observations 

 show conclusively that this is quite wrong. 



