SARGEXT : THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 137 



the optic lobes of Petromyzon. Johnston (:0l) described this nidulus 

 in Acipenser as the 'nucleus magnocellularis.' EJiuger (:0l), in a 

 brief description of the nidulus in selachians, applied the term 'nucleus 

 magnocellularis tecti ' and reaffirmed Osborn's error by maintaining its 

 connection with the trigeminus. 



The homology of the large-celled nidulus of the mesencephalic roof 

 with the cells of the torus longitudinalis has remained unsuspected. 

 No one of the investigators cited, with the exception of Edinger, who 

 saw the connection of the cerebellar ueurites with the cerebellum, has 

 traced any of the neurites correctly. 



In December, 1899, I described before the Society of American 

 Morphologists at New Haven the origin of the fibre of Reissner as the 

 result of the fusion of the axous of the large cells of the mesencephalic 

 nidulus, and published a brief account the following May (Sargent, :00^). 

 At the Baltimore meeting of the same society (December, 1900) I an- 

 nounced the discovery of "An Apparatus in the Nervous System of 

 Vertebrates for the Transmission of Motor Eeflexes arising from Opti- 

 cal Stimuli," showing the relation of the mesencephalic large-celled 

 nidulus to the fibre of Eeissner, and describing its development and 

 physiology, a brief account of which was published the following April 

 (Sargent, :01, :0r). 



Houser (:01) has since confirmed the anatomical details of my work 

 as then announced, so far as they relate to selachians. 



B. Methods and Material. 



In spite of the distinctness and constancy of this fibre in the prepa- 

 rations where it was first seen, its unusual position made me at first 

 incredulous as to its being a preformed structure. My first care, there- 

 fore, was to make certain that it was in no way due to the coagulation 

 of the cerebro-spinal fluid, or the formation of other artifacts by the 

 particular fixing agents used. 



As already stated, it was first observed in the trout, which had been 

 fixed in corrosive sublimate. Knowing the proneness of corrosive sub- 

 limate to form artifacts closely resembling organic structures, I searched 

 for the fibre in material fixed in fluids containing no corrosive sublimate. 

 It was found to be equally clear and sharp in material fixed in Flem- 

 miiig's fluid, in formol, and in many other fluids unlike corrosive 

 sublimate in their action. 



The fixing fluids were chosen so as to secure as great a diversity of 

 composition as possible, in order to test all possible effects in the coagu- 



