SECONDARY SPECTACLES 453 



differently felted from what they are deeper in the corneal thickness; 

 but a vertical section through the corneal thickness shows no line of 

 demarcation in the substantia propria. 



In the fishes, however, it is extremely common to find such a line, and 

 to find that the fresh cornea can readily be peeled apart along the in- 

 ternal boundary surface which the line represents (Figs. 67 ^ 105a; pp. 

 159, 261). It is apparently this incompleteness of fusion between the 

 original cornea and its dermal addendum which has made it easy for 

 many fishes and some amphibians to produce 'secondary' spectacles, 

 which actually represent a regression to the anatomical condition in the 

 lampreys. Even in the highest vertebrates, the corneal epithelium oc- 

 casionally remembers all too well its origin as head-skin epidermis. Sheep 

 have been known to exhibit a cornea completely covered with wool. 



Secondary Spectacles — These are definitely associated in many cases 

 with the habit of coming out of water into dry air, or of groping for food 

 on a sandy or muddy bottom. Secondary spectacles occur in practically 

 all amphibious fishes, and in a host of bottom species. The secondary 

 spectacle is never homy like a tertiary one, however, and cannot offer a 

 cornea so good a protection against desiccation. Moreover, since many 

 bottom-feeding fishes have small, poorly developed eyes, it is impossible 

 to say which small-eyed forms have split off a spectacle from the cornea 

 as a positive adaptation to serve a special purpose, and which possess a 

 spectacle as an embryonic arrest, as an evidence of a tendency of the eye 

 to degenerate. For cave salamanders, it is particularly easy to say that 

 the adult has a spectacle because the degenerate eye has been halted in 

 an embryonic condition — the primary spectacle never becomes a part of 

 the eyeball in cave forms as it does, at metamorphosis, in other salaman- 

 ders. Too little is known of the mode of development of secondary spec- 

 tacles in fishes — certainly many arise through an embryonic failure of 

 fusion rather than a secondary splitting of the cornea after fusion. Such 

 spectacles would be secondary only in the sense of a phylogenetic delami- 

 nation of the cornea, the fishes having superimposed an inhibition upon 

 the fusion-tendency which their ancestors permitted to operate. There is 

 however some suggestion that many piscine conjunctivae are fused and 

 later separated during development, in the fact that there are usually 

 some connective-tissue strands crossing from the surface of the residual 

 cornea to the inside of the spectacle — such very tenuous and elastic 

 strands (Fig. 152a, st) that the spectacle is able to remain motionlessly 



