540 CHORD ATA. 



usually fibrous, but in many fishes a cartilaginous, layer, which in 

 front becomes transparent and strongly curved, forming the cornea. 

 The second layer, the choroid coat, is richly vascular and pig- 

 mented; at the boundary between sclerotic and cornea it is changed 

 to the iris. The inner layer is the retina, the structure and 

 arrangement of which are characteristic of the vertebrates. 



From the developmental standpoint the retina (fig. 82) con- 

 sists of two parts, the retina proper and the tapetum iiigrum 

 (pigmented epithelium), formerly regarded as part of the choroid. 

 In the retina the following layers are distinguished: (1) the limi- 

 tans interna; (2) nerve-fibre layer; (3) ganglionic layer; (4) inner 

 molecular layer; (5) inner granular layer; (6) outer molecular 

 layer; (7) outer granular layer; (8) limitans externa; and (9) layer 

 of rods and cones. The limitans externa is the bounding mem- 

 brane of the embryonic retina, which is later penetrated by the 

 rods and cones. Between the two limiting membranes Miiller's 

 fibres (m) extend, large supporting cells occurring in other sensory 

 epithelia, the nuclei of which lie in the inner granular layer, and 

 which are aided in their supporting function by the fine horny ' 

 framework of both molecular layers. The nervous elements 

 which are imbedded in this support are best understood by begin- 

 ning with the optic nerve. This spreads out in the nerve-fibre 

 layer, and on its way to the end apparatus comes twice into relation 

 with ganglion cells; first in the ganglionic layer, second in the 

 inner granular layer. Thus a great part of the retina (layers 1 to 

 6) are to be considered as an optic ganglion, such as occurs in 

 molluscs and arthropods, but which there lies outside the sensory 

 apparatus. The sensory epithelium (the retina in the sense this 

 term is used in invertebrates) consists of but two layers, the- 

 outer granular layer and the rods and cones. The outer granules 

 are the nuclei of the extremely slender epithelial cells which bear 

 the rhabdomes (rods and cones) on their peripheral ends. Pigment 

 cells are lacking between these visual cells, but the pigment so 

 necessary for the visual function is supplied by the tapetum 

 nigrum already mentioned. This is a layer of hexagonal epithelial 

 cells which lies on the tips of the rhabdomes and sends pseud opodia- 

 like processes between them, and since the tapetum is rich in 

 black pigment granules, the rods and cones are enveloped in a 

 close pigment mantle. 



Although in this relation of pigment and in the union of the 

 optic ganglion with the sensory cells important differences are to 

 be noted from the eyes of the invertebrates, even of the closely 



