Embryology of the Retina of Teleosteans. 267 



retina. Though longer than any others in the retina, these 

 cells are as yet comparatively short ; their nuclei are large. 

 These represent the future retinal sight-epithelium, a layer of 

 rods and cones with their nuclei. The inner mass of cells 

 remains for a short time unchanged, except that midway they 

 are more loosely placed, indicating faintly a line of separation 

 between its outer and inner halves. Between this cell- mass 

 and the future cone-layer there is a distinct space, corre- 

 sponding to the position of the late vesicle-cavity and of the 

 future outer molecular layer. Internally the retina is indis- 

 tinctly bounded by a line, sliowing the foundation of the 

 layer of nerve-fibres, for fibrillation soon becomes distinct in 

 it near the optic nerve entrance. In the cells previously 

 described as occurring between the optic outgrowth and 

 brain, and which now appear (on section) long-oval and 

 flattened horizontally, pigment-granules begin to form. 

 When first visible these granules closely surround the cell- 

 nucleus only, the remainder of the cell being free. The pig- 

 ment forms first in the cells at the anterior and lower part of 

 the fundus of the eye, and in those cells in front of the peri- 

 phery of the lens which correspond in position with the future 

 iris. In one or two preparations this pigment-layer was 

 accidentally turned over to some extent in cutting the sections, 

 and we thus obtain a surface view ; the cells are now seen to 

 be of good size, flat, with large nuclei, and arranged as a 

 pavement-epithelium. About this time pigment also begins 

 to appear here and there over the brain, es))ecially anteriorly, 

 occurring in the flat cells previously described. Both here 

 and round the retina therefore the pigment is formed in cells 

 derived, I believe, from the deep layer of the cuticular epi- 

 blast, not, in the case of the Teleostean eye, from the inner 

 wall of the optic vesicle. To be confident on this point, 

 however, further observations are necessary, and especially a 

 complete series of preparations of ova of one species, a desider- 

 atum that can only be satisfied by a marine laboratory such 

 as that of St. Andrews. 



A finely granular layer (internal molecular) next appears 

 on the large cell-mass, dividing it into a smaller inner and 

 larger outer part. At first both sets of cells are about the 

 same size ; but tliose internal to the inner molecular layer 

 soon become distinctly larger than those outside it; tiie 

 former represent the ganglion cell-layer, the latter the layer 

 of inner granules. The outermost cells of the inner series 

 very soon become distinct from the others, staining more 

 deeply and being arranged in a regular single row. About 



