50 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
18. There is a notable sulcus on the lateral side of the adult optic 
lobe, which increases the surface area of the tectum. 
19. The nidulus corticalis is the origin of the horizontal commissure 
and of a large bundle of nerve fibres which pass into layers 3 and 4 of 
the tectum. 
20. The most important receiving cells for the fillet layer are proba- 
bly the large spindle cells in layer 4. 
