300 ARTHUR WILLET. 



and Julin in the case of Clavelina; and although it might 

 appear at first sight that somebody must be right, it turns out 

 in fact that all are wrong. I have stated above that a com- 

 munication between the cavity of the nervous system and 

 stomodajum is effected in the simple Ascidians at a consider- 

 ably later stage than that described by Kowalevsky. Kemem- 

 bering the general abbreviation in the development of Clave- 

 lina, to which attention has already been directed, we should 

 expect to find that this communication would become estab- 

 lished at a much earlier stage in Clavelina than, for instance, 

 in Ciona, and this is exactly what happens. 



2. Origin of the Neuro-hypophysial System in Ciona 

 intestinalis. 



Stage I. — In transverse sections through a young tadpole 

 which has, by convulsive movements of its tail, succeeded in 

 bursting the egg-follicle, and so entered upon the brief free- 

 swimming phase of its existence, we find that quite anteriorly 

 a minute portion of the cavity of the cerebral vesicle is 

 separated ofi" from the main cavity, and appears in section as 

 an independent lumen in the thickness of the wall of the cere- 

 bral vesicle on its left side (fig. 6). A section or two behind 

 this the small lumen in question is seen to communicate freely 

 with the cerebral cavity (fig. 7), and farther back still there is 

 nothing more of it to be seen but simply the plain wall of the 

 vesicle (fig. 8). 



Thus already at this stage a portion of the cerebral vesicle 

 has begun to be constricted off" in the form of a tube, at present 

 ending blindly in front, and communicating behind with the 

 main cavity of the vesicle. It is of a very snjall extent, and lies 

 at present entirely in the thickness of the wall of the vesicle ; in 

 fact, it is only with the utmost pains that it can be detected at 

 all at this stage. Attention may be drawn here to the cupule 

 of the otolith shown in fig. 7 in the ventral wall of the cerebral 

 vesicle. The otolith itself has become separated from the 

 cupule which normally carries it, in the process of cutting. A 

 distinct nucleus-like body is to be seen in the interior of the 



