20 



THE PINEAL ORGAN 



lying mesenchyme form a transparent layer termed the cornea ; or in 

 some cases this layer is called " secondary cornea," whereas the unpig- 

 mented superficial segment of the optic vesicle is called the " primary 

 cornea " (Fig. 36, Chap. 3, p. 50). When a simple upright eye has only a 

 single lens it is classed as an " ocellus," a term also applied to eye spots 

 and other simple types of eye. The lens of an invertebrate eye may be 

 non-cellular or cellular. The non-cellular type is most frequently formed 

 by a thickening of the cuticle or chitin, — cuticular lens ; sometimes as a 

 condensation of the secretion of vitreous cells (Figs. 33-35, 38, Chap. 3, 

 pp. 48, 49, 53). The vitreous may be formed from a secretion of the cells 

 lining the optic vesicle, but in some cases it appears to originate from two 

 or more very large cells in the cavity of the optic cup or vesicle, which 



Fig. 15. — Longitudinal Vertical Section through the Parietal Eye of an 

 Advanced Embryo of Anguis fragilis showing Distinct Boundary 

 between Lens and Retina. (After J. Beard, Q.J. Micro. Sc, 29, 1889.) 



/. : lens. ret. : retina. 



degenerate and give rise to a clear viscous fluid, as in the eyes found on the 

 back oiOnchidium (Fig. 39, Chap. 3, p. 54). 



Cellular lenses may be formed in invertebrates by elongation of the 

 hypoderm cells, as in the median eyes of the blow-fly (Fig. 9) or the ocelli 

 of the larva of a beetle (Dytiscus) which shows a transition from the 

 surrounding hypoderm cells through the elongated " vitreous " cells of 

 the lens to the sensory cells of the retina. This type of lens is frequently 

 combined with a cuticular lens which forms a transparent thickening of 

 the cuticle over the centre of the " vitreous " part (Fig. 4, p. 10). A cellular 

 lens formed from the distal wall of the optic vesicle is well seen in the 

 median or " pineal eyes " of some lizards, e.g. in the blind-worm — Anguis 

 fragilis and Sphenodon (Fig. 15 ; and Fig. 183, Chap. 20, p. 259). A good 

 example of a cellular lens is also present in the remarkable eyes arranged 

 round the edge of the mantle in the Scallop (Patten) (Fig. 107, 



