STUDIES ON THE PROTOCHORDATA. 307 



In Ciona, therefore^ the cerebral ganglion of the adult arises 

 by proliferation and constriction from the dorsal wall of the 

 neuro-hypophysial tube. 



3. Origin of the Neuro-hypophysial System in 

 Clavelina lepadiformis. 



The description given above as to the origin of the neuro- 

 hypophysial system in Ciona, together with that which I am 

 about to give for the same system in Clavelina, will be found 

 to be considerably at variance with the results obtained by 

 Seeliger and van Beneden and Julin in the case also of 

 Clavelina. 



My observations were at first entirely confined to Ciona, 

 and led me to the conclusion, judging from the very explicit 

 account, accompanied by numerous figures, of van Beneden and 

 Julin (loc. cit,), that the mode of development of the parts in 

 question must be different in Clavelina. But when I came to 

 study the origin of these structures in the latter form to 

 enable me to make a definite comparison with Ciona, it turned 

 out that the relations above described for Ciona were not only 

 essentially the same in Clavelina, but were very much easier to 

 determine, on account of the larger size of the object. 



The stage which van Beneden and Julin took as their point 

 de depart was much older than that which I shall now 

 commence with. In fact, their first stage was that at which 

 the larval nervous system had already attained the climax of 

 its development. 



Stage I. — The stage from which I find it is desirable to start 

 in describing the future development and fate of the nervous 

 system of Clavelina is a very young embryo, with the anterior 

 neuropore still open to the exterior; no mouth, no atrial 

 involutions, no pigment in the brain, and before the migration 

 of the otocyst. 



A transverse section through the neural tube, some distance 

 behind the neuropore, is shown in fig. 25, PI. 11. The part of 

 the neural tube extending between the region through which 



