Integrative Systems 



149 



and two median. The retina, the essential nervous layer of the 

 paired eyes possessed by all vertebrates, develops as a hollow ventro- 

 lateral outgrowth from the posterior region of the forebrain (Figs. 

 135B, 141) The outgrowth shapes into 

 a cuplike vesicle connected with the 

 brain by a relatively narrow optic 

 stalk, along which the fibers of the 

 optic nerve later develop. Accordingly, 

 in the adult, the optic nerves join the 

 brain in the floor of the diencephalon 

 (Figs. 137, 140). 



The median eyes develop from the 

 i oof of the diencephalon. Just behind 

 the choroid plexus are usually formed 

 two small, hollow, external projections, 

 one close behind the other (Fig. 138). 

 The anterior one is called the parietal 

 organ; the other is the pineal organ. 

 Either is sometimes called an "epiph- 

 ysis." Either or both of them may 

 produce at its distal end a small eyelike 

 organ with pigmented retina, lens, and 

 a nerve passing down into the deeper 

 part of the diencephalon (Fig. 142). 

 When thus well developed, the eye lies 

 at or near the surface of the head, 

 and the skin covering it is thin or 

 transparent. 



In lamprey eels, both the parietal and the pineal eye are fairly well 

 developed. The parietal eye is especially characteristic of lizards, in 

 which it lies in a parietal foramen between the parietal bones and is 

 covered by a transparent scale. The pineal organ is prominent in shark- 

 like fishes and in toads and frogs, but not strongly eyelike. In adult 

 birds and mammals, it appears as the "pineal gland," a small non- 

 nervous lobe closely attached to the roof of the diencephalon (Figs. 

 139, 140), apparently glandular but of uncertain function. In what 

 way and to what extent these median eyes on top of the head are use- 

 ful to their possessors is quite problematic. 



The floor of the diencephalon, just behind the place where the optic 

 nerves enter it, projects downward as a hollow conic structure, the 

 infundibulum, which bears at its tip a small solid lobe, the pituitary 

 body (hypophysis), a highly important endocrine gland (Fig. 140). 



The ventricle of the diencephalon (diacoele, or "third ventricle" 



Fig. 141. Stereogram of the 

 developing eye. The head of the 

 embryo is cut transversely in the 

 region of the forebrain. (<•/"! 

 Choroid fissure ; {fb) wall of fore- 

 brain; (/) ectodermal thickening 

 which invaginates to form lens; 

 (oc) optic cup; {os) optic stalk; 

 (p) outer thin wall of optic cup, 

 becoming the pigmented epithe- 

 lium which lies behind the defini- 

 tive retina; (r) inner thick wall of 

 optic cup. becoming the sensory 

 retina of the eye. (Courtesy, 

 Kingsley: "Comparative Anat- 

 omy of Vertebrates," Philadel- 

 phia, The Blakiston Company.) 



