333 



At the time of the fixation of the larva however this communica- 

 tion between the nerve-tube and the stomodaeum certainly exists 

 and it is precisely this communication which van Beneden and 

 Jul in emphatically deny. It is easy to demonstrate the opening of 

 the neural tube into the stomodaeum in sagittal sections, such as that 

 represented in the accompanying figure which is taken from a young 

 individual of Ciona intestinalis just after fixation. 



As is well-known, the cerebral vesicle of the Ascidian larva atro- 

 phies during the metamorphosis, but before it does so, a portion of its 

 wall becomes constricted off from the vesicle as a tube which opens in 



opening of neural tube 

 into mouth 



cerebral vesicle 

 (filled with 



histolytic residua) 



branchial epithelium 



ganglion 



neural canal 



mesoderm 

 (Drawn with cam. lue. Zeiß J. immersion. Oc. 3. 



front into the mouth, while behind it is continuous with the rest of 

 the neural canal. The constriction of a tube from the wall of the cere- 

 bral vesicle was described by van Beneden and Julin, but the 

 Hypophysis was described as a thing apart, arising purely and simply 

 by an evagination from the wall of the branchial sac and par excellence 

 distinct from the nervous system. As shown however in the above 

 figure, the Hypophysis and the neural tube are at first one and the 

 same thing and it is only at a later period that the definitive Hypo- 

 physis and the ganglion become difi"erentiated and separated from one 

 another. The first indication of the formation of the ganglion from 



