SEGMENTAL BILATERAL SYMMETRY 233 



for example, the supreme motor centres are situated in the 

 cortex of the right and left cerebral hemispheres. From 

 these centres the right and left motor tracts pass down 

 through the substance of the brain, converging as they do 

 so, to the upper part of the spinal cord called the medulla, 



Fiq. 72. — Correction of deviation in the evolving Pish, k-sa 

 is the straight line of attraction leading to sa the source ; p. q, 

 are the left and right sides of the developing brain ; A, B, the 

 left and right lateral fins. In position (1) tike attracting force 

 affects side Q of the brain more strongly than side p, and in 

 demanding movement forwards into the line e-sa can only 

 obtain this by obliging the left fins, A, to drive strongly. If 

 side p is unaffected by the force at first, the swinging of the 

 Fish into r— sa soon brings P under sa's influence, when the right 

 fins, B, also drive. And they do so with increasing power until, 

 when the long axis of the Fish is in the line e-sa, both sides 

 of the brain are equally affected by sa, and the symmetrical 

 drive of the right and left fins produces straight forward move- 

 ment towards sa. A similar drawing could be made with the 

 Fish subjected to force of repulsion from an antero-laterally 

 situated source R. In this case the Fish would be attracted to 

 a posteriorly laterally situated source of attraction, sa, and 

 have a greater distance to swing round to move in the line 

 leading from B to sa. 



where, anteriorly situated, they are visible as the " pyramids." 

 Immediately below the pyramids the majority of the con- 

 ducting fibres of the right side cross over to run down the 

 left side of the cord, while the majority of those on the left 



