DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 



151 



a spherical mass of cells, solid in some cases (fig. 60 d) but with a cavity in 

 others. It is still connected with the skin. In one case the lens was a vesicle with 

 a distinct epithelium bounding the cavity (fig. 60 e). In the other cases there 

 seemed to be no regularity in the arrangement of the lens cells. 



The pigmented layer has become very thin compared with the thickness of the 

 rest of the retina. Its thickness increases toward the margin of the cup. The 

 retina is very thick, with about 5 layers of nuclei ; these are crowded except at the 

 free margin of the retina, which is free from nuclei. There is no histological differ- 

 ence between the different cells of the retina unless there is an appreciable elonga- 

 tion in the cells at the margin of the cup. 



Optic fibers are not yet developed. 



cplsng. 



elhpig 



tns. 



Ins. 



Fig. 60. 



(a) Transverse Section of Eye of Cymatogasttr larva, 3.2 mm. long. 



(b) Transverse Section of Eye of Cymalogastcr larva, 4.5 mm. long. 



(c) Transverse Section of Eye of Amblyopsis embryo, 4.4 mm. long. 



\d) Section of Right Eye of Larva. 4.4 mm. long. Nuclei all drawn without a change of focus. 



(c) Vertical Section of Eye of another Larva, 4.4 mm. long. 



FIVE-MILLIMETER STAGES. 



The embryo is hatched at the beginning of this period. The least differentiated 

 eye of this stage is represented in vertical section in figures 61 a and b. The second- 

 ary vesicle has become more definitely formed. The vitreous cavity is reduced in 

 size and the retina has become distinctly thicker, but shows as yet no differentiation 

 into different layers. 



