THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 175 



epithelium has become thin and is composed of a single series of cells. The eye, at 

 this stage, does not differ materially from the eye of a Cymatogaster larva about half 

 as long (compare Figs. 27, 28). 



There is no indication of a differentiation of an iris. The secondary cup is a 

 shallow, bowl-shaped structure, the depression being entirely filled by the thickening 

 of the skin which is giving rise to the lens (Figs. 25, 27). 



2. Four-millimetre Stages — I n specimens 4.4 millimetres long the eye had 

 become a deeper cup than it was during the 3-millimetre stage. The lens, which no 

 longer fills the entire cavity, has become a spherical mass of cells, solid in some cases 

 (Fig. 32) 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. 33). 

 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 five layers of nuclei ; these are crowded except at the 

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

 between the different cells of the retina unless there is an appreciable elongation in 

 the cells at the margin of the cup. 



Optic fibres are not yet developed. 



3. Five-millimetre 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 34 and 35. The secondary 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. 



In a larva 5 millimetres long the eye is still in contact with the epidermis on one 

 side and the incipient dura mater on the other. The epidermis is distinctly thinner 

 over the eye, reaching an extreme thinness of 16 micra as compared with a thickness 

 of 40 micra at a distance of 100 micra below the eye and of .24 micra at 100 micra 

 above the eye. 



The lens lies directly beneath the skin. In this particular eye (PI. XV ; Fig. 36) 

 it is an ellipsoid, 30 micra by 38 micra (36 by 28 in another eye) . It is entirely sepa- 

 rated from the skin and takes on a deeper stain. The cells of the lens are not very 

 regularly grouped, but apparently they are arranged about a median point or space. 

 The lens lies entirely outside of the eye in contact with the outer face of the dorsal 

 part of the iris. 



The eye proper is a subspherical solid mass with only a shallow depression below 



