88 BULLETIN OF THE 
vitreous body of the eye, and is, as just shown, a modified portion of the 
hypodermis, with which it has never ceased to be continuous. 
The cells of the middle (inverted) layer undoubtedly form all of the 
retinal elements. Iam unable to assert positively what becomes of the 
“inner layer.” Certain stages show that the nuclei of this layer have 
become considerably flattened in the direction of radii to the optic bulb, 
so that it is possible they ultimately constitute a kind of enveloping tunic 
to the deep surface of the bulb. That I have not mistaken mesodermic 
elements for this posterior layer, is evident from the great size of the 
nuclei, and the successive stages exhibited in the conditions of the layer. 
It, therefore, seems to me at present doubtful if the inner layer really 
shares in the formation of the retina proper. After the completion of 
the infolding the cells of the middle layer elongate and the layer thus 
becomes thicker, especially its central portion, and the whole invaginated 
mass, therefore, assumes a more nearly spherical form. This elongation 
of the cells appears to result from an outgrowth of their anterior ends, 
since the nuclei are crowded into the deeper portions of the layer, while 
the anterior part becomes translucent. The formation of the bacilli in 
the extreme anterior ends of the cells soon makes this part of the layer 
more highly refractive. The bacilli increase in length from before back- 
wards. A little later a zone of pigment granules makes its appearance in 
the retinal cells between the forming bacilli and the more anterior of the 
nuclei. These granules are limited to near the surfaces of the cells. 
About the same time a layer of pigment cells is observable along the 
deep surface of the eye-bulb. They are probably the cells of the “inner 
layer” of the involution, for they do not appear to be elongated in the 
direction of the axis of the bulb, as are the nuclei of the true retinal or 
nerve-end cells. As the pigmentation increases it appears in some of the 
retinal cells in a position posterior to the prenuclear zone above men- 
tioned ; these patches of pigment seem to correspond in height with the 
elongated nuclei of the cells, and are not always continuous with the 
prenuclear zone of pigment. 
It is probable, therefore, that not only the posterior ends of the bacilli 
are practically ensheathed by a layer of pigment, but also that the nuclei 
of the nerve-end cells become more or less enveloped in pigment, and 
that the two zones are not at first continuous with each other, nor with 
the still more posterior pigmentation of the inner layer. The pigment in 
all cases belongs to cells of the originally involuted ectoderm, and there 
are no interstitial cells between the nerve-end cells of the retina; at least 
satisfactorily stained specimens show only nuclei of two sorts: namely, 
