CONCLUSION. 159 



4. The characteristic shape of the arthropod eye and the arrangement of its 

 retinal cells is retained in an exaggerated form in the vertebrate retina, and 

 affords us the only satisfactory explanation of its inversion, its contour and 

 mode of growth, its choroid fissure, its arrangement of rod and cone cells, and its 

 centrally located optic nerve. 



5. The parietal eyes of vertebrates belong to the second forebrain neuromere, 

 the lateral eyes to the third or fourth. 



6. The optic lobes of primitive vertebrates represent the compound eye 

 ganglia inverted and transferred to a position overlying the mesencephalic neuro- 

 meres. Their genetic relations, as well as their most intimate functional and an- 

 atomical relations, are with the procephalic neuromeres. 



7. The ganglia habenulae of vertebrates represent the ganglia of the parietal 

 eyes of arachnids, united in the middle line over the region of the diencephalon. 

 They were primarily associated with the olfactory lobes. 



