450 Avery E. Lambert, 



By means of this process the optic thickening comes to be turned 

 toward the ganglia of the brain ; the retinal elements being literally 

 turned upside down, which accounts for their inverted position in 

 the adult eye. 



The result of this growth of the fold is the formation of a sac-like 

 vesicle which is open in the posterior direction. The fold, itself, 

 forms the roof of this vesicle, while the floor consists of the optic 

 and cerebral ganglia of the brain (text-fig. o, B; v. c). The lumen 

 of this sac is continuous with that of the anterior optic vesicle. (Teixt- 

 fig. 3, B; lu.). 



As the growth of the cerebral lobes continues, they overlie more 

 completely the vesicle formed by the anterior optic invagination, the 

 lumen of which is finally closed by the thickening of its walls, and by 

 the pressure which is brought to bear upon it from above. (Text- 

 fig. 3, C and D; a. o. v.). 



The formation of the nerves supplying the anterior median eyes 

 has been accounted for in two ways ; first, as outgrowths from the 

 optic tracts of the brain, and, second, as elongations of the retinal 

 cells which become secondarily attached to the optic lobes. 



There is a third way which a careful examination of my material 

 has led me to believe may account for the formation of the optic 

 nerves. As the growth of the optic fold progresses, its inner layer, 

 connecting the retinal thickening with the dorsal lip of the optic 

 vesicle, comes to lie close upon the optic lobes, and finally unites with 

 them (text-fig. 3, B ; o. n.). The lower portion of this layer becomes 

 separated from the upper portion by a process of delamination, and 

 is turned ventralwards l)y the continued growth of the optic lobes. 

 The upper portion of this layer maintains its connection with the 

 retinal cells in the optic plate, and with the dorsal Avail of the anterior 

 optic invagination ; the cells of this portion of the middle layer of 

 the fold elongating as the retina is carried farther away from the 

 anterior invagination by the growth of the fold, forming the fibers 

 of the optic nerve. 



I cannot claim that I have established the fact that the nerves 

 of the anterior median eyes arise in this fashion ; but careful and 

 repeated examination of my material leads me to believe that these 



