222 LESLIE B. AREY 



occupied (hiriiij;- light adaption. In this type of experiment, 

 therefore, it is also evident that severance of the optic nerve 

 restrains the normal pigment migration. The fact that the pig- 

 ment of excised eyes does not contract in darkness, and that 

 pigment contraction in general seems to involve a less vigorous 

 response than does expansion, explains why consistent results 

 were obtained in all these experiments, for the competition with 

 the inhibiting mechanism presumably was less keen. 



To remove any doubt that the integrit}^ of the optic nerve is 

 necessary for the migration of pigment in light and in darkness, 

 the nerve was cut inside the cranium near the chiasma. In order 

 to do this, a small aperture was made in the cranium over the 

 region of the optic nerve and the severance of the nerve was 

 accomplished by introducing a cataract scalpel through the open- 

 ing thus made. The removal of bone from the thick cranium 

 had to be done with care or the fish failed to recover from the 

 resulting shock effect. In successful experiments the results 

 obtained were similar to those previously recorded. Since the 

 eye and the surrounding parts were left intact during the whole 

 procedure, it is safe to conclude that in Ameiurus the optic nerve 

 is intimately related to the phenomenon of pigment migration. 



A microscopical study of preparations demonstrating the rela- 

 tion of the optic nerve to the nerve-fiber layer of the retina shows 

 that the arrangement of nerve fibers is approximately of a radial 

 nature, and that the fibers from any sector of the retina are 

 extended into the adjacent side of the optic nerve. These rela- 

 tions probably become disturbed a short distance from the 

 emergence of the optic nerve from the eyeball. The optic nerve of 

 Ameiurus joins the retina by several roots (fig. 7, rdx. n. opt.), 

 hence this condition is not shown as diagrammatically as in 

 preparations of the eye of Abramis which cut the optic nerve 

 in a radial plane. Here the nerve fibers form a distinct V-shaped 

 ^parting' as they pass to the two sides of the retina. On the 

 periphery of the nerve this is especially evident although at the 

 center there is undoubtedly more decussation, as w^as shown by 

 Kohl ('92) for several of the lower vertebrates. 



