THE ELASMOBRANCH FISHES 



269 



cones (co.) and rods (rd.) which are intimately associated with vision. These 

 occupy the layer farthest removed from the lens and next to the epithelial 

 lining (ep.). The rods and cones together with the cells from the first or gan- 

 glionic layer may be described more fully. 



The rods (rd.) and cones (co.) are characterized by the extreme length of 

 the cell body. The nuclei of the rods extend almost to the heavy outer layer, 

 and the slender cells reach the epithelial lining. The cones are heavier but are 

 fewer in number with their nuclei staining as dark bodies also in the heavy 

 outer layer. The cone cells extend toward the outer epithelium, but unlike the 

 rods they do not reach the outer layer. 



It is the axones from cells of the retina that pass to the brain as the optic 

 nerves. 



Development of Eye 



For convenience the development of the eye may be considered in two parts : 

 (1) the development of the retina and its associated parts; and (2) the de- 

 velopment of the lens. 



The first indication of the eye makes its appearance as a slight down-pitting 

 of the cephalic plate even before the plate closes (see fig. 209, op.v.). Upon its 

 closure these pits are directed 

 outward toward the ectoderm, fl 



as the optic vesicles. The optic 

 vesicles are therefore direct 

 outgrowths from theforebrain. 

 As each vesicle nears the ecto- 

 derm it sinks in at its outer 

 margin forming a tw^o-layered 

 cup, the stem of which as the 

 optic stalk binds the bowl of 

 the cup to the brain. The outer 

 layer of the bowl does not de- 

 velop as nerve tissue but be- 

 comes pigmented. The choroid coat develops back of this. The inside (invagi- 

 nated) layer thickens to form the retina, the fibers from which pass along the 

 old optic stalk to the brain as the optic nerve. 



The first indication of the lens is seen as a thickening of the ectoderm at the 

 place where the optic vesicle touches it and before the vesicle invaginates to 

 form the optic cup. The lens then becomes spherical, pinches off from the ecto- 

 derm, and sinks into the cup. 



Fig. 235. Sagittal section through fenestra to ear, 

 Haja. (From Howes.) A. Median sagittal. B. Para- 

 sagittal. 



e.d., endolymphatic duct; fl., fluid; fn., fenestral 

 tube; 7nZ>., tympanic membrane; ^y., tympanic cavity. 



ACCOMMODATION APPARATUS 



In almost all the Elasmobranchs. excepting types like Squatina, the eyes 

 are so far separated that it would be impossible for both eyes to focus on a 

 given point at the same time. In all these, binocular vision is impossible and 

 monocular vision is the rule. Little adjustment of the lens is here possible. It 



