' SAEGENT : THE OPTIC KEFLEX APPARATUS OF VERTEBRATES. 205 



PoECiLiiDAE (92). In adult Fundulus heteroclitus Reissner's fibre is 

 1.0 micron in diameter, and has been followed through its whole course. 

 The torus longitudinalis is well developed, especially in the anterior part 

 of the mesencephalon, and hangs suspended freely in the mesocoele 

 below the level of the tectum. 



In larvae 1 cm. long, the median roof anterior to the posterior com- 

 missure (the ' Schaltstuck ' of Burckhardt) remains in a state of primi- 

 tive simplicity (Plate 8, Fig. 59, prs. t^cal.). As in many other species, 

 the cells of the torus are arranged in radiating columns, separated by 

 ependymal fibres (fo7'. Ig.). The fibre-like bundles of axons (M. for. fhr. 

 Reis. a.) coming from the anterior part of the torus pass between its 

 spindle-shaped ependymal cells (eras, e'end.) to enter the ventricle, where 

 they unite to form the greater portion of Reissner's fibre (Fig. 59, /^r. 

 Eeis.). 



There is no marked ventriculus terminalis, but in the slightly dilated 

 posterior extremity of the canal is centralis there is a chain of about a 

 dozen small posterior canal-cells. 



Ambltopsidae (93). This group includes a number of species of 

 small cave-inhabiting fishes having degenerate eyes ; some of them are 

 totally blind (see Eigenmann, '99, ;03). The four species examined^ 

 afford a series in which the effect of disuse of the visual organs is shown 

 by profound changes in the central nervous system (compare Figs. 

 G and H), accompanied by the degeneration of the optic reflex 

 apparatus. 



Chologaster papilliferus has functional eyes, greatly reduced in size, 

 the maximum diameter being 0.8 mm. Tlie optic lobes are relatively 

 much smaller than in the closely related Fundulus, which has normal 

 eyes. 



The tectum is thin and the torus-lobes but little developed. Reiss- 

 ner's fibre has in the adult a diameter of approximately 0.6 micron, 

 being relatively much smaller than in the i-elated Fundulus. 



The three other species are totally blind. In Amblyopsis the ves- 

 tigial eye has a maximum diameter of 200 micra; in Typhlichthys, 180 

 micra ; and in Troglichthys, 85 micra (Eigenmann, '99). In the two 

 latter species I have been unable to distinguish the slightest trace of 

 Reissner's fibre. The optic centres of the central nervous system of 

 Amblyopsis have recently been studied by Ramsey (:0l), who finds the 



1 I am greatly indebted to Prof. C. H. Eigenmann for tlie privilege of examin- 

 ing his extensive collection of preparations of blind fishes. He also kindly pro- 

 vided me with fresh material of Chologaster and Amblyopsis. 



