88 BULLETIX OF THE 



vitreous body of the eye, and is, as just shown, a modified portion of the 

 hypodermis, with Avhich it lias never ceased to be continuous. 



The cells of the middle (inverted) layer undoubtedly form all of the 

 retinal elements. I am 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 thai 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 comjjletion 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 bidb, 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 jiractically 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. Tlie 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. 



