CHAPTER XVI 

 OPTIC AND VISUAL-MOTOR SYSTEMS 



THE eyes in Amblystoma are much better developed than they 

 are in Necturus, where they are degenerate, though functional. 

 The retina of Triturus is more highly differentiated, and vision is 

 evidently more efficient. In frogs this advance is carried much far- 

 ther, with corresponding structural elaboration of the visual centers 

 of the tectum opticum and thalamus. 



OPTIC NERVE AND TRACTS 



The optic tracts and their connections of Necturus have been 

 described ('41a). An account of the development of the optic nerves 

 and tracts of Amblystoma was published in the same year ('41), and 

 subsequently ('42) a detailed analysis of the visual centers and their 

 connections, together with commentary on related physiological 

 problems. Some further theoretical considerations were published 

 later ('44a). These details need not be repeated. The salient features 

 were outlined in chapter iv, and some topics are amplified here. 



In the coil stage (Harrison's stage 34), optic nerve fibers begin to 

 grow from the ganglion cells of the retina through the optic stalk 

 before the other retinal layers are histologically differentiated ('41, 

 p. 477; Coghill, '16, Paper II, p. 274). In the S-reaction stage (Harri- 

 son's stages 35, 36) these fibers reach the brain; in early swimming 

 stages (Harrison's stages 36, 37) they decussate in the chiasma ('41, 

 p. 480); and shortly thereafter they reach the tectum (Harrison's 

 stage 38) and the primordial basal nucleus in the peduncle. Up to this 

 stage, rods and cones and other retinal layers are incompletely dif- 

 ferentiated. In stages immediately preceding effective feeding reac- 

 tions (about stage 44) the retina is functional, and the larva can 

 orient itself with reference to an object moving in the field of view; 

 and shortly thereafter well co-ordinated lurching and snapping move- 

 ments efficiently capture moving prey. The first visual responses are 

 probably activated by fibers from the tectum through the posterior 

 commissure to the nucleus of Darkschewitsch and the fasciculus 

 longitudinalis medialis and perhaps also by the basal optic tract, 



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