VISUAL TISSUES ^53 



cesses from connective-tissue cells which lie far below the basal mem- 

 brane and the visual cells. These processes often contain intra-cellular 

 blood capillaries, and sometimes the connective-tissue nucleus is found 

 in them. The line of contact between an accessory cell and such a 

 connective-tissue process is always on the basal membrane. 



The visual cells in the adult have their principal cell body and nucleus 

 far below the basement membrane (Fig. 223, A, B). The proximal end of 

 each is produced into a nerve fiber, and the distal end, just above the 

 accessory retinal cells, gives rise to a long, wide, and round-ended visual 

 rod. The most important structure in the eye is the neuro-fibril, which 

 enters the cell body and passes through the cell to enter into the visual 

 rod as an axial filament. Here it traverses the whole length of the rod 

 in a somewhat sinuous course and terminates as a knob in the tip. The 

 relations of the pigmented cytoplasm of the accessory retinal cells to this 

 axial filament and end-knob are most interesting. When in the dark, 

 the pigment is all to be found in the pigment band, which marks the distal 

 ends of the accessory cells. As the light increases, the pigment-bearing 

 cytoplasm of these cells travels in a thin stream that surrounds the fibril 

 until it reaches the knob (Fig. 223, A, represents an intermediate 

 stage). When the light is brighter, as in direct sunlight, the knob itself 

 is surrounded and appears as a brown lump. This last condition is 

 seen in a single rod from Octopus, pictured as Figure 223, B. 



We should speak here of the remarkable condition found in the tetra- 

 branch cephalopods. The eye of nautilus is much like the squid's as to 

 retina, but all the complex accessory apparatus is wanting. No lens or 

 cornea is present, and yet the well-developed retina has an image pro- 

 jected upon it by the way its mouth of invagination is arranged as a 

 pin-hole camera. Through this .hole the image is formed as in a 

 camera lucida. The eye-sac is full of sea water and in constant com- 

 munication with the water. 



As Nautilus is a more rudimentary form, it may be asked if this 

 represents a more primative condition. It may, as it is analogous at 

 least to such eyes as those of Patella and Trochus. But we must remem- 

 ber that the surviving members of ancient races frequently show 1 marked 

 degenerations which amount to simplifications, and it is quite possible that 

 Nautilus shows such a condition in its eye. Its retina suggests the latter. 



The mammalian eye, which represents to a degree all vertebrate eyes, 

 is not only as complex histologically as the cephalopod eye, but it is more 

 highly organized and is probably the most efficient eye in existence. In 

 its derivation from the embryonic tissues it is unique, and we shall under- 

 stand it best by studying it from that point of view. 



The most distinctive feature of its development consists in the fact 

 that two principal tissues take part in forming it jointly. The retinal 



