276 



CHARLES E. STOCKARD 



pushed out. Figures 8 illustrates diagrammatically a cross sec- 

 tion of this condition. In the course of development the fibers 

 of the optic nerve following the stalk reach the lateral position 

 and must enter the brain and continue within its tissue in order 

 to meet the nerve of the opposite side and form the cross or 

 chiasma. Brain tissue would lie beneath the optic chiasma and 

 the chiasma would necessarily be within the brain. This con- 

 dition is never found in any normal vertebrate. 



Fig. 8 A diagrammatic cross section through the brain and optic cups in an 

 imaginary case in which the optic connections are lateral with median brain tissue 

 originally lying between the eye anlagen. The future optic fibers gr-owing along 

 the optic stalks as paths reach the lateral points on the brain from which the 

 stalks arose and then enter the brain tissue before having formed the cross. 

 Continuing to grow, the optic nerves meet and cross in the median plane. The 

 cross is within the brain itself and lies above the median mass of tissue which 

 has always existed between the eyes. No vertebrate brain exhibits such a con- 

 dition. 



One might claim that the optic fibers on reaching the brain 

 ran ventrally and formed the cross beneath, but no such change 

 in direction is seen at any stage of their development. The 

 structural relationships seem to depend upon a median origin 

 and connection of the optic stalks. 



The fact that a cyclopean eye may have no well formed optic 

 stalk and is entirely median in position not necessitating the 



