210 



The optic-cup remains in a primitive condition. The inner layer 

 is not directly opposed to the outer, there being a distinct interval 

 between the two. The inner layer shows the more or less clearly 

 marked retinal elements; yet, owing to the character of the fixation, 

 it was impossible to thoroughly study the histological details. 



The thickness of the retina varies greatly in different specimens 

 as can be seen by reference to Figures 1 to 7. In order to obtain 

 an average measurement for the thickness of the retina of each eye, 

 four measurements were taken at an equal distance from the entrance 

 of the optic nerve. The mean of the four measurements was then 

 computed. In a minimum case this was 0,075 mm, while in an eye 

 whose retina was especially thick, the mean amounted to 0,176 mm. 

 The mean thickness of the retina of the average eye amounted to 

 0,131 mm. 



There is a fairly close resemblance between the right and left 

 «yes of any given specimen not only as regards the thickness of the 

 retina but in the shape and position of the eye-ball as well. 



In many cases the portion of the retina ventral to the entrance 

 of the optic nerve is decidedly thinner than the region above it. This 



Fig. 8. 



Fig. 9. 



Fig. 10. 



Fig. 11. 



possibly may be associated with an incomplete growth in the region 

 of the choroid fissure. In some instances the choroid fissure remains 

 permanently open (Fig. 6). 



The outer layer of the optic-cup is composed of a single layer of 

 unpigmented cubical cells. This single layer is continuous around the 

 rim of the optic-cup, the inner layer of which may have the same 

 histological character as the outer layer for a greater or less distance 

 from the rim; see Figures 3 to II, all drawn to scale. I have inter- 

 preted this thin rim as either a rudimentary or vestigial iris, cor- 

 responding to the retinal layer of the iris in the other vertebrates. 



