THE VISUAL APPARATUS. 



145 



these theoretical questions should add in- 

 terest to the study of the development and 

 nerve connections of the visual apparatus. 

 It was shown in an earlier chapter that 

 the lateral eyes are developed as a pair of 

 vesicles from the dorso-lateral wall of the 

 brain in about the region of the second 

 neuromere (Figs. 18-22). Each vesicle 

 comes to be attached to the ventral wall of 

 the brain by a hollow stalk. There now ap- 

 pears in the ectoderm a thickening which 

 sinks in to form a pit and finally separates 

 from the ectoderm as a closed sac. This 

 then develops into the lens of the eye. 

 Meanwhile the optic vesicle becomes cup- 

 shaped toward the outside, enclosing the 

 lens, and the wall of the concave side 

 thickens to form the retina, while the con- 

 vex wall remains thin. For the further de- 

 velopment and adult relations of the eye-ball 

 and accessory structures the reader must be 

 referred to the larger text-books of embry- 

 ology and anatomy. The histogenesis of 

 the retina proceeds in a manner similar to 

 that of the spinal cord or brain. Supporting 

 structures are formed (the so-called Miillerian 

 fibers) which are to be compared vdth the 

 spongioblasts of the brain, and among these 

 are found neuroblasts which give rise to the 

 nerve cells of the retina (Fig. 71 A). On 

 the inner surface of the retina, that is next 

 to the portion of the brain ventricle which 

 extends into the optic vesicle, there are formed 

 spindle-shaped cells whose one process ex- 

 tends to the ventricular surface and ends in 

 a rod-shaped or cone-shaped structure, and 

 whose other process runs deeper into the 



Fig. 72.— Cells from 

 the retina of the chicken. 

 From Cajal. a, b, centri- 

 fugal fibers. 



