THE OPHIDIAN EYE 631 



modation (see pp. 282-3).* Since the back of the lens rests solidly 

 against the cupped vitreous, it would not seem to matter whether the 

 anterior limiting membrane of the latter flares out to attach to the ciliary 

 roll (forming a 'posterior leaf for the 'zonule') or coincides — as appar- 

 ently it often (or usually) does, with the anterior leaf. Where two leaves 

 are discriminable, there are practically never any other zonule fibers to 

 be seen between them, with attachments to the equatorial region of the 

 lens. A conspicuous exception however is Epicrates. 



All snakes have a plexus of tiny blood vessels on the inner surface of 

 the retina,"^ fed by an artery which enters through the optic nerve, and 

 drained by the nasal and temporal arcs of the hyaloid vein (lying on the 

 orbiculus) into a mid-ventral trunk which passes back over the surface 

 of the retina to leave the globe through the optic nerve. This vein and 

 artery are clearly homologous with those which, in lizards, supply the 

 conus papillaris. And, in scattered members of every good-eyed family 

 of snakes, they supply a conus as well as a network of vitreal vessels. 

 But the 'conus' of these snakes has no genetic connection with the conus 

 or pecten of other sauropsidans, for its framework consists of meso- 

 dermal connective tissue — not of neuroglia. It is never large or dagger- 

 like, but most often forms a low mound, pigmented or clear, with a 

 brush of cuticular fibers which emanate from it in all directions to 

 disappear in the vitreous. It is longest and slenderest in Vipera berus 

 (where it is heavily pigmented) and Lampropeltis triangulum (where it 

 is colorless) ; but it is never relatively longer than the conus of a noc- 

 turnal lizard. The history of the ophidian 'conus' is best illuminated by 

 the fact that it is frequently much larger in embryos than in their adults, 

 and is often present in embryos whose adults lack all traces of it. More- 

 over, the development of the vitreal vessels goes hand in hand with the 

 ontogenetic retrogression of the conus : as the latter dwindles, the meso- 

 derm of its flared base creeps out on the surface of the retina, centrif- 

 ugally from the optic nerve head, and it is in this film of mesoderm that 

 the hyaloid plexus takes form. The embryological history of the conus- 

 artery is thus strikingly like that of the mammalian hyaloid (see p. 113 

 and Fig. 42a, p. 112). 



*In life, the two leaves of the zonule diverge much more, toward the lens, than they are 

 shown doing in Figure 181 (p. 628). 



tin Tdrbophis, these vessels are really embedded in the retinal tissue — some of them, quite 

 deeply, as in mammals. 



