ANATOMY AND DEVELOPMENT. 223 



At the stage shown in Fig. 105 A, the lumen of the 

 alimentary canal is extremely reduced, and in many places, 

 as in the region of the endostyle, e, its opposite walls are 

 in actual apposition, so that the lumen at these points is 

 almost obliterated. 



This temporary reduction of the lumen of the alimentary 

 canal is due to the narrow space into which it has to be 

 compressed, combined above all with the relatively enor- 

 mous size of the cerebral vesicle, which exercises a great 

 pressure on the subjacent dorsal wall of the branchial sac. 

 It may be added that the larva of Ciona does not take in 

 food independently until after fixation. 



Reopening of Ncnropore ; Degeneration of Cerebral Vesicle; 

 Formation of Definitive Ganglion. 



One of the most obvious features of the metamorphosis 

 is the rapid expansion undergone by the enteric and body 

 cavities and the no less rapid degeneration of the cerebral 

 vesicle. This expansion, by relieving the crowded char- 

 acter of the various parts, facilitates greatly the study of 

 the changes which take place in the internal organisation. 



The neuropore, which we have described above as having 

 closed up at an early period, now reopens again and places 

 the neural tube that is to say, as much of it as remains 

 after the atrophy of the tail --in open communication 

 with the base of the buccal tube (Fig. 105 B, ;/). 



The spacious cavity of the cerebral vesicle has vanished, 

 and its walls have undergone disintegration, and, except 

 for a portion of the dorsal wall which becomes converted 

 into another channel, are now represented by a mass of 

 histolytic residua filling the original cavity of the vesicle 

 and lying below the anterior portion of the nerve-tube. 



