RELATION OF MEDIAN TO LATERAL EYES 365 



sentatives — showing that the brain, cranial nerves, vestibule, and semi- 

 circular canals along with the branchial and vascular systems were in 

 essential points alike — fully warrants the assumption that the lateral 

 eyes of the extinct ostracoderm fishes and living Cyclostomata were 

 essentially alike. In other words, that the lateral eyes of the cephalaspid 

 and other fossil fishes were of the inverted type found in living species, 

 and that they were functional. Moreover, judging from the small size 

 of the parietal foramina or impressions in many of the extinct Palaeozoic 

 fishes as compared with the size of the orbital cavities ; and that in many 

 cases the parietal canal did not even pierce the roof of the skull, it must 

 be concluded that the lateral eyes were the chief organs of vision, and 

 that in those cases in which the parietal canal did not pierce the vault of 

 the skull, the parietal eye was completely functionless as a visual organ. 



The replacement of the simple ocellar eyes of the larva by the com- 

 pound eyes of the imago has been clearly demonstrated in the water 

 beetle, Dytiscus marginalis, by Giinther, and has already been alluded 

 to, p. 117. It will be necessary, however, to describe in detail the 

 differentiation of the crescentic or kidney-shaped area of epithelium 

 called the " optic plate " or " rudiment of the lateral eye " in order to 

 compare the ommatidia of the lateral eye with the ocelli which precede it 

 (Figs. 78, 79, 80, 81, Chap. 11, pp. 11 7-1 19). The cells of the optic plate 

 elongate and also proliferate and those in the centre of the plate become 

 grouped into cylindrical unit-systems or retinulce. Their protoplasm 

 becomes clear, and some of the nuclei assume a deeper position. Each 

 group consists of eight cells, one central and seven peripheral. The central 

 cell and six of the peripheral cells take part in the formation of a retinula, 

 while the seventh cell of the peripheral series is pushed out of the system. 

 The central cell, which is flask-shaped in its basal portion where the 

 nucleus is situated, tapers into a fine rod as it approaches the surface. 

 The remaining six cells with the central basal cell form the visual portion 

 of the retinula or rhabdome. Superficial to each retinula and between it 

 and the ectoderm are especially modified clear ectoderm cells containing 

 vacuoles, which give rise to the crystalline cones and together with these 

 form the ommatidia. Each crystalline cone is formed by the amalgamation 

 of four clear rods and each of these rods is derived from a refractile vesicle 

 which is contained in one of the clear ectoderm cells. Later the four 

 rods cohere and give rise to a crystalline cone, as in the crustacean, 

 Palcemon (Fig. 37, Chap. 3, p. 52). The cells between adjacent ommatidia 

 extend through the entire thickness of the ectoderm and secrete pigment, 

 while superficially the cuticle, which covers the whole area, is secreted 

 by small corneagen or lentigen cells. The cuticular layer is thickened over 

 the distal ends of each ommatidium to form a plano-convex cuticular lens. 



