224 THE ASCIDIANS. 



This remnant of the cerebral vesicle of the larva with its 

 sense-organs becomes eventually absorbed, and the eye and 

 otoltth may often be found floating about the body-cavity 

 with the ordinary mesenchyme-cells, and occasionally they 

 can be seen actually passing through the heart. 



The anterior portion of the nerve-tube itself, which now 

 opens into the base of the buccal tube or stomodceum,* is 

 derived from a portion of the dorsal wall of the original 

 cerebral vesicle which was constricted off from the latter in 

 the form of a narrow tube slightly to the left of the mid- 

 dorsal line (Fig. 105 B, n). 



Subsequently the cells forming the dorsal wall of this 

 portion of the nerve-tube proliferate and form a solid 

 thickening which becomes the definitive ganglion of the 

 adult (Figs. 105 C, 106, and 107, g). 



The lumen of the nerve-tube behind the region of 

 the definitive ganglion finally becomes obliterated by the 

 mutual approximation of its constituent cells, and that 

 portion of the primitive nerve-tube which in the larva lay 

 between the cerebral vesicle and the root of the tail is thus 

 represented in the adult by a solid "cordon ganglionnaire 

 visceral" (van Beneden and Julin) which starts from the 

 posterior end of the adult cerebral ganglion, and, proceed- 

 ing along the dorsal side of the pharynx above the dorsal 

 lamina, becomes lost among the viscera. (Cf. Figs. 96, 

 105, and 107.) 



Below and in front of the definitive ganglion, which 

 finally becomes quite separate from the dorsal wall of the 

 neural tube, the lumen of the latter persists and becomes 



* According to renewed observations on Ciona, I find that the neuropore 

 reopens into the buccal tube precisely in the line of junction of the stomo- 

 dceum with the wall of the branchial sac, so that its upper margin is continu- 

 ous with the (ectodermic) stomodoeal epithelium, and its lower margin with 

 the (endodermic) branchial epithelium. (See below, V.) 



