414 



CHORDATE ANATOMY 



the chorioid fissure, divides it along its lower side, and extends as a 

 groove along the lower side of the optic stalk. (Fig. 362, c/) 



Of the two layers of the optic cup, the outer persists as the single- 

 layered pigment epithelium, and the inner thickens to form the three 

 layers of neurons of the definitive retina. Neurites from the inner 

 ganglion-cell layer grow along the optic stalk and gradually fill its groove. 

 Half of these fibers cross to the opposite side of the brain, forming the 

 chiasma of the optic nerve. The cells of the optic stalk are converted 

 into neuroglia cells. 



PARAPHYSIS SUPER K 



A. CYCLOSTOME ^S^MissfiRE 



EPIDERMIS-^ 



--CORIUM " 



- SKULL 



MIDBRAIN 

 -PINEAL NERVE 



PARAPHYSIS 

 DORSAL SAC 



PINEAL TRACT 



B. ANURAN 



PARIETAL EYE 



PINEAL ORGAN 



CORIUM ^EPIDERMIS 



PARIETAL NERVE 



PINEAL TRACT 



SUPERIOR 



HABENULAR' 



COMMISSURE 



■^ MIDBRAIN 



i POSTERIOR COMMISSURE 



DORSAL SAC- 



SUPERIOR HABENULAR/ 



COMMISSURE 



POSTERIORI 

 COMMISSURE 



MIDBRAIN 



C. REPTILE 



D. MAMMAL 



Fig. 365. — Parietal and pineal organs of vertebrates as seen in median longitudinal 

 section. A, cyclostome; B, anuran; C, reptile; D, mammal. Both parietal and pineal 

 organs have served as median eyes. In amphibians it is the pineal organ, in reptiles the 

 parietal organ. In man and mammals the pineal organ becomes a gland of doubtful 

 function. (Redrawn from Oppel, after Studnicka.) 



The remaining parts of the adult eye are derived from mesenchyma. 

 The cellular elements of the aqueous humor, and possibly also some of 

 those of the vitreous body, are mesenchyme cells which enter the optic 

 cup by way of the optic fissure. But the fact that fibers connect the lens 

 and retina has led some investigators to infer that the vitreous body is 

 derived from the retina and therefore is ectodermal. On the outside of the 

 eye, the chorioid and fibrous tunics are added from the surrounding mesen- 

 chyma. The eye muscles of man have a like origin, although in lower 

 vertebrates they arise from the walls of the head "cavities." Folds of 

 skin form the eyelids, which unite temporarily but in man separate 

 before birth. 



