52 T. H. MORGAN. 



bands running from without in, through what has been spoken of 

 as the zones. These zones of the middle layer are not of course 

 separated from each other, but pass insensibly into each other, 

 and are only of use in describing the structure of the middle layer. 



The nuclei found in the middle layer lie largely to the right and 

 the left of the middle line of the eye, and through quite a wider 

 zone of this layer. Thus they show to a large extent a bilateral 

 arrangement. The inner layer is surrounded on its entire inner 

 surface by a pigmented layer — the inner and pigmented layer of 

 the eye. This layer is composed of pigment granules, both black 

 and yellow, and it is impossible in such sections to resolve the 

 layer into its constituent cells. The entrance of a part of the 

 optic nerve is seen in the figure. It disappears in the pigment 

 layer in the inner middle line, and cannot be traced farther, in 

 sections, than the pigment layer. 



Fig. 33 is a drawing of a longitudinal section through the 

 same eye. It is therefore at right angles to the last figure and 

 along the line of the raphe. This section, however, is not 

 median through all its length, but at the outer part passes to one 

 side of this line; at the inner part, however, it cuts approxi- 

 mately the middle line. But this slight obliquity is really advan- 

 tageous to an understanding of the eye, as it passes more in the 

 plane which the principal retinal elements follow. The three 

 layers of the eye are clearly seen, although the distinction 

 between the outer and middle layers is not so definitely marked 

 as we saw in the cross-section, and is due here to the oblique 

 plane of the section. Under the corneal hypodermis lies the 

 large middle layer, in which we see the same structures as shown 

 in the cross-section of the eye. The nuclei are conspicuous and 

 numerous, and much more abundant here than in a section 

 passing through the exact middle line. There are large vacuoles 

 lying amongst the nuclei, but I have been unable to determine 

 whether these are artefacts or natural to the eye. But I can 

 scarcely believe them to be artificial products, as they seem to 

 be constantly found on this part of the eye, and show no evidence 

 of being due to tearing of the sections. 



The layer of rounded bodies is not shown in the figure. The 

 form of the retinal elements is better seen in this section than in 

 the cross-section. At the inner part of the middle layer is a 



