226 THE ASCIDIANS. 



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 dona. 



The convexity caused in the dorsal wall of the branchial 

 sac by 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 epi bran- 

 chial groove of JULIN (Fig. 97). At present it is merely a 

 ridge, the epibrancJiial ridge. 



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

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

 the peripharyngeal band, which we have already described 

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

 cialisation of the cells forming the epithelial wall of the 

 pharynx at this point. 



Primary Topographical Relations and Change of Axis. 



It must be especially 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 larva for the purpose of 

 describing the primary topography, 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 embryogenetic 

 processes differ in many respects widely from what occurs 

 in Amphioxus, we are inevitably compelled to rely to a 

 very large extent on topographical relations in order to 

 estimate the homology of this or that structure in the 



