222 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



laterally in the tectum breaks up in its ectal portion among the endings 

 of the retinal fibres of the optic nerve. The tractus toro-cerebellaris 

 turns laterally below the dorsal decussation and eventually takes an 

 oblique course around the lateral border of the tectum to the cerebellum. 



The third set of processes, the axons, have a generally anterior and 

 ventral direction, eventually entering the ventricle and by fusion forming 

 the fibre of Reissner. In the more primitive teleosts the nerve-fibres 

 enter the ventricle in the anterior portion of the median fissure between 

 the torus-lobes. In embryonic and early stages the axons enter the 

 ventricle more or less singly, but as development advances they become 

 consolidated into a smaller and smaller number of fascicles, in which the 

 constituent axons are no longer distinguishable. 



In the more highly diff'erentiated teleosts the mode of formation of 

 Reissner's fibre is more complex. The anterior portion of the optic lobes 

 and torus project cephalad of the posterior commissure and overlie the 

 diencephalon. The torus-cells of this region send their axons, as the 

 tractus toro-fibrae Reissneris anterior, by the shortest path into the ven- 

 tricle, through the ependymal thickening of the pars intercalatus. Axons 

 of cells in other portions of the torus take the more primitive path to 

 the ventricle and constitute the tractus toro-fibrae Reissneris posterior, 

 which enters the ventricle in the narrow recess between the lobes of the 

 torus posterior (dorsad) to the posterior commissure. 



Some fine branches coming from the base of the ganglia habenulae run 

 caudad through the diencephalon in the ependymal groove below the 

 ' Schaltstuck.' Caudad of the posterior commissure all these tnmks 

 unite to form the fibre of Reissner, which runs posteriorly through the 

 central canal, and gives off branches to the cord through the posterior 

 two-thirds of its course. At its posterior extremity it is in connection 

 with a system of posterior canal-cells, which are smaller but more 

 numerous than in selachians. 



II. Physiological. 



A. Comparative Physiology. 

 1. The ' Flight Reflex: 



The physiological action of this optic reflex apparatus has been ob- 

 served for many years, though nothing was known of the apparatus by 

 which these reactions were brought about. Edinger ('99, p. 279) says : 

 "The conduct of the youngest brood of fish, still attached to the 



