TERTIARY SPECTACLES IN REPTILES 



455 



of a circular lid fold — just as in any land vertebrate — this fold gradually 

 closes in over the eye, the aperture surrounded by it shrinking to the 

 vanishing point and moving dorsally the while (Figs. 153, 154). Thus it 

 is manifestly the lower lid which contributes the greater part of the spec- 

 tacle. In the rattlesnake, the lid opening even becomes a normal hori- 

 zontal palpebral fissure before it closes, like a healing wound, leaving 

 no scar or trace in the finished goggle. In one snake, Rbinopbis, a small 

 slit is still present in the newborn young. Though it is now certain that 

 the nictitating membrane does not even start to develop at all in the 





Fig. 153 — Embryological formation of spectacle in a snake, Natrix tiatrix. 

 After Schwarz-Karsten. 



A circular lid-fold grows, in over the cornea, its aperture at first large and concentric (upper 

 left) but shrinking and taking up a dorsad position (lower right), eventually closing com- 

 pletely before birth. The finished tertiary spectacle thus comprises chiefly the lower lid, 

 the upper lid making only a small contribution and the nictitans none at all. 



snake (let alone form the spectacle), it is still a puzzle that the tear- 

 gland associated with the upper and lower lids, the lacrimal, should be 

 absent in snakes while the one which lubricates the third eyelid in other 

 vertebrates — the oily Harderian gland — should be present. The Harder- 

 ian secretion flows into the space between the deUcate one-layered cor- 

 neal epithelium and the spectacle, and drains through a duct into the 

 nose, then into the mouth to mingle with and supplement the saliva. 

 It is possible that the fluid has a high refractive index and some optical im- 

 portance, but the optics of the tertiary spectacle remain to be worked out. 



