SARGENT. — REISSNER'S FIBRE. 



449 



The axis cylinder runs caudad in the median plane just ventral to the 

 floor of the canal. I venture the conjecture that this cell represents, in 

 Ainphioxus, the tectal cells of craniotes, and that its giant axis cylin- 

 der is homologous with Reissner's fibre. In the phylogeny of this appa- 

 ratus the cyclostomes would then furnish the connecting link between 

 Amphioxus and the gnathostomes. 



A number of experiments have been performed to determine the func- 

 tions of this apparatus. The best results have been obtained with dogfish 

 (Squalus acanthias), and small sharks (Charcarias littoralis). In these 

 animals Reissner's fibre is of such size (10 p, to 20 p. in diameter) that 

 it is possible to remove portions of it from the brain ventricles or canal 

 for microscopical examinations. The operation of breaking the fibre 



*<&** 



fbr n . opt. 

 eiopl rpc. 



Figure A. Diagram of the optic reflex apparatus, cbl., cerebellum ; cl.can. p., 

 posterior canal cells ; cl. opt. rfx., optic reflex cells ; cl. P., Purkinje cells of cere- 

 bellum ; fbr. n. opt., afferent optic nerve fibres ; fbr. R., Reissner's fibre ; vnt. trm., 

 ventriculus terminalis ; tct. opt., tectum opticum. 



was easily performed. By an incision through the integument, and the 

 removal of a small bit of cartilage, the tela choroideus covering the 

 fourth ventricle was exposed. Through this a curved needle was plunged, 

 and drawn several times transversely across the ventricle to break the 

 fibre. The wound was then dressed ; the animal, when replaced in the 

 water, usually recovered from the sbock in from twenty to thirty minutes. 

 Those individuals in which the operation had been performed without 

 success in breaking the fibre, served for the control experiments. The 

 success or failure was ascertained at the autopsy, performed at death or 

 on the fourth or fifth day after operation. Furthermore, the cord and 

 medulla of each individual was preserved for microscopical examina- 

 tion. In about one third of the cases the operation had failed to 

 vol. xxxvi. — 29 



