SEGMENTAL BILATERAL SYMMETRY 235 



sources somewhere behind ; (3) from sources directly to the 

 right or left. 



As examples of the first we shall take the optical sensory- 

 paths. 



In Fig. 73 there is represented the anterior end of a 

 developing primitive Fish, and the main directions in which 

 light waves would pass to the brain from objects situated 

 anteriorly. 



Now our suggestion is that the rays of light have 



Fig. 74. — Diagram of the course of the fibres in the human 

 optic tracts, R, right eye ; L, left eye ; o, c, crossed tracts ; 

 s, s, straight tracts. The crossed tract of the right eye, 

 coming from the nasal half of the right retina, passes through 

 the chiasma to run to the left hemisphere in company with 

 the straight tract of the left eye, coming from the temporal 

 half of the left retina. The dotted lines suggest fibres crossing 

 over between the corpora quadrigemina (cq), or further back 

 towards the cortex, lh, eh, left and right hemispheres. 



demanded sensory paths to lead them to the brain, and also 

 demanded that these paths should follow the direction the 

 rays themselves habitually take ; and that as a result of 

 this a crossed sensory optic tract has inevitably been formed 

 in the typical Fish. In the next figure the plan of the human 

 optic chiasma is given for comparison (Fig. 74). 



The next diagram deals with tactile impressions from 

 sources situated behind the brain, and we can see how in 

 every case the strongest waves the brain would receive 



