16 ARTHUR DENDY. 



function which Sargent assigns to it. The question of ciliation 

 must be left for future investigation to settle, but Sargent has 

 evidently misunderstood my observations on this subject, for 

 he makes me say that the cilia of the grooves are longer than 

 those of the ventricular walls generally, whereas I both de- 

 scribed and figured them as being much shorter. What I 

 described as cilia are, therefore, probably not the same struc- 

 tures as Sargent describes as constituents of Reissner's fibre, 

 though I now believe myself that they may possibly indicate 

 merely a striated margin of the columnar epithelium. My 

 recent observations show, however, that in the Velasia stage 

 of Greotria it is possible to make out extremely fine threads 

 (fig. 6, R.F.'") projecting from the epithelium of the epen- 

 dymal groove, much longer than the supposed cilia, and these 

 are in all probability the nerve fibrils described by Sargent. 

 Sargent maintains, as already stated, that these fibrils are 

 connected with, and in fact go to make up, the fibre of 

 Reissner, and he regards the whole system as a short circuit 

 for optic reflexes. He has found Reissner's fibre, with 

 similar relations, throughout the entire vertebrate phylum, 

 and brings forward experimental as well as histological 

 evidence in support of his views. 



In Greotria Reissner's fibre (fig. 2 and 6, R.F.) is con- 

 spicuously developed, and has in most respects the same 

 relations as described by Sargent in Petromyzon. It 

 appears to originate in the immediate neighbourhood of the 

 ependymal groove, beneath the posterior commissure, and is 

 made up of a number of branches (fig. 6, R.F.', R.F.") which 

 can be traced close up to the columnar epithelium of the 

 groove.^ Though I have not been able actually to demon- 

 strate the connection between this epithelium and Reissner's 

 fibre, I see no reason to doubt the correctness of Sargent's 

 statement as to the existence of such a connection by means 

 of the delicate fibrils which emerge from the epithelium. 



' Tiie two grooves ia Geotria are so closely approximated as to form 

 practically a single groove (fig. 3, Ep.G.), the form of wliicli, however, 

 clearly indicates its double origin. 



