274 CHARLES R. STOCKARD 



In the ordinary individual the future eye forming material is 

 first located in a median position in the medulla^-y plate. This 

 material becomes more extensive or widens laterally, and two 

 growth centers are established, the material between the centers 

 finally becomes the median ventral layer of cells of the optic 

 stalks. Later the incipient optic vesicles begin to evaginate or 

 push in a ventro-lateral direction and finally turn dorsally and 

 laterally to reach their usual places at the sides of the head. 

 The optic stalks, however, still lead back to the ventro-median 

 position and there in the fish the optic fibers, following the optic 

 stalks as paths, cross and in higher vertebrates form the optic 

 chiasma always in the ventro-median plane below the brain 

 floor and from here the optic tracts proceed to their centers in 

 the brain. 



The median position of the optic chiasma outside and below 

 the brain is an important structural fact in the present consid- 

 eration. Figure 6 is a diagram of a transverse section through 

 an early brain with the optic cups in their usual position. The 

 optic stalks connect with the brain and the median ventral cell 

 layer is actually part of the stalks. As development proceeds 

 the optic fibers arising from the cells in the retina follow along 

 the optic stalks to reach the brain. The investigations on the 

 development of the optic nerve have shown that the optic stalks 

 become solid and form the supporting paths or neuroglial scaf- 

 folding along which the optic nerve fibers grow. The fibers from 

 one retina meet those from the other in the median plane below 

 the brain and in the fish the fibers from the two retinae cross 

 directly while in higher forms partial crossing takes place and 

 the optic chiasma is formed outside the brain. Figure 7 is a 

 sketch representing a cross section of the actual condition of 

 the early optic nerves in a fish embryo. This position of the 

 optic cross is only possible if the median tissue be optic stalk 

 tissue. 



• Suppose, on the other hand, that the eyes primarily originate 

 from lateral medullary tissues and between the two eyes other 

 brain tissue is present. The optic stalks are then attached to. 

 the lateral regions of the brain from which the optic vesicles 



