THE CHELONIAN RETINA 611 



represented (as in fishes, amphibians, and other reptiles) by a broad 

 funnel whose mouth coincides roughly with the retinal ora terminalis. 

 Instead, there is a slender canal of fairly uniform diameter which runs 

 forward from the disc, like the mammalian canal of Cloquet, but does 

 not reach and touch the lens; rather, it ends on the anterior hyaloid 

 membrane toward the temporal side (Fig, 175). 



The vascular pattern of the eyeball, as worked out by Fritzberg on 

 Emys orbicularis, compares quite closely with that of the frog. There are 

 no vitreal vessels, however — their place was probably taken (physiolog- 

 ically) in primitive reptiles by the conus papillaris (p. 607) ; but no 

 well-developed conus occurs in any known adult chelonian. In advanced 

 embryos of Chelonia, Chrysemys, and Chelydra, a small, unpigmented, 

 avascular glial cone forms upon the nerve head; but in the adults of 

 these genera (except perhaps Chelonia) , the surface of the 'disc' smooth- 

 ly continues that of the surrounding retina. It is difficult to say why the 

 turtles have been able to dispense with (or to avoid evolving?) a conus 

 when the lizards have not, for the turtle retina is nearly as rich in cones; 

 but the general difference in activity of turtles and lizards is perhaps 

 the explanation (see p. 653). The poor development of the average 

 chelonian chorioid strongly suggests that the metabolic requirements of 

 the retina are relatively low. 



The Retina — The retina is decidedly impure in its lamination (Fig. 

 176a), with every nuclear layer containing some elements which 'belong' 

 at some other level. The horizontal cells have ropy processes, and may 

 have reverted completely to a non-conductive function. All or nearly all 

 chelonians have an area centralis. Outside of this, the visual: ganglion 

 cell ratio is in the neighborhood of 2:1; but within the area there is of 

 course a lower summation-ratio. A fovea has been claimed, and later 

 authoritatively denied, for each of several genera; but such a feature has 

 been convincingly demonstrated (by photomicrography) only in Amyda 

 (by Gillett, who failed to realize the uniqueness of his discovery) . 



Prior to 1877, about everyone who described a chelonian retina saw 

 rods in it, but since that time, owing to one of the few mistakes (and 

 the weighty authority) of Max Schultze, the turtles have been placed 

 among the pure-cone reptiles. They do however possess droplet-free ele- 

 ments with heavy, cylindrical outer segments, morphologically identical 

 with the unquestionable (rhodopsin-containing) rods of crocodilians 

 and with the plump peripheral rods of birds. It is not known whether 

 these cells contain rhodopsin, but since they are most numerous in the 



