226 THE ASCIDIAXS. 



of the neuro-hypophysial tube while the latter is still in 

 connexion with, and therefore before the atrophy of, the 

 cerebral vesicle, thus indicating a hastening in the devel- 

 opment as compared with Cioita. 



The convexity caused in the dorsal wall of the branchial 

 sac bv the pressure of the cerebral vesicle persists as the 

 anterior portion of the dorsal lamina, and in many or most 

 simple Ascidians becomes grooved, forming the cpibran- 

 chial gj-ooz-c of Julix (Fig. 97). At present it is merely a 

 ridge, the cpibraiichial ridge. 



In Fig. 105 r the proximal (oral) end of the endostyle, 

 c, is seen to be connected with the epibranchial ridge by 

 the pcripliaryngcal band, which we have already described 

 in the adult. It apparently arises in situ by simple spe- 

 ciahsation of the cells forming the epithehal wall of the 

 pharvnx at this point. 



Primary Topograpliical Relations and CJiange of Axis. 



It must be especiallv noted that the long axis of the 

 young Ciona for some time after fixation is identical with 

 that of the tailed larva, and therefore the primary topo- 

 graphical relations of the various parts are maintained at 

 the stage shown in Fig. 105 B, and we can accordingly make 

 use of this stage in which different structures are much 

 clearer than in the free-swimming lan^a for the purpose of 

 describing the primarv topographv, which is of the utmost 

 importance when it is desired to institute a comparison 

 with Amphioxus. 



Since, as we have seen, the details of the embr\"ogenetic 

 processes differ in manv respects widelv from what occurs 

 in Amphioxus, we are inevitablv compelled to relv to a 

 ver^■ large extent on topographical relations in order to 

 estimate the homologv of this or that structure in the 



