176 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



the lens representing the vitreous cavity and choroid fissure. In the eye more par- 

 ticularly described here the depression is filled largely with blood-corpuscles (Fig. 

 36, cpl. sng.). 



The pigmented layer is not more than 4 micra thick, and is very sparingly pig- 

 mented over the posterior face of the eye. At the iris and the lower margin of the 

 choroid fissure it is continuous with the inner layers of the retina through cells whose 

 nuclei are distinctly elongated. The retina proper, from the pigmented layer to the 

 vitreous cavity, is 64 micra thick. 



It is differentiated into a nuclear layer (the outer and inner together) and the 

 ganglionic layer, separated by the incomplete inner reticular layer. 



The ganglionic layer is composed of two sorts of cells. Those nearer the vitreous 

 cavity have much more distinct nucleoli than those nearer the reticular layer. 



Cell multiplication is still going on. 



The optic nerve is well developed, forming a solid strand of fibres, 12 micra in 

 diameter, readily traceable to the brain. 



The muscles are represented by strands of cells closely crowded. No striation 

 is evident. 



4. Six-millimetre Stages — In embryos 6 millimetres long the cells giving rise to 

 the oblique muscles and those for at least two of the recti can be distinguished. 

 Scleral cartilages are not yet formed. 



In three of the specimens sectioned there was no indication of a lens. In others 

 it was well developed. Cell division was still going on in the retina. 



The optic vesicle was very shallow. The rim of the vesicle was wide and still 

 continuous with the choroid fissure, which showed as a shallow groove along the ven- 

 tral surface. The choroid fissure, instead of leading into a central secondary optic 

 cavity, led to the mass of ganglionic cells (Fig. 38). This condition of the choroid 

 fissure and its relation to the interior of the eye leads me at this point to say a few 

 words concerning the general structure of the eye. In the description of the eye of 

 the adult I considered that the central ganglionic mass was the result of the collaps- 

 ing of the eye with the disappearance of the vitreous body and cavity. I was justified 

 in this conclusion by the process of degeneration going on in the eye of Typhlomolge, 

 Typhlichthys, and Typhlogobius. Whatever may have been the phylogenic process 

 in Amblyopsis, it is evident that ontogenically the mass of cells does not arise as 

 imagined. It appears from the embryos that the condition of the adults arises more as 

 the result of a contracting of the retinal area without a corresponding decrease in the 

 size of the eye as a whole than as the result of the collapsing of a vesicle followed by 

 the coalescence of the walls brought together by the collapse. Sagittal sections of 



