118 THE GENESIS OF THE VERTEBRATE EYE 



A' in the growing eye until metamorphosis, when throughout the re- 

 mainder of the much-expanded retina ('Retina B') the visual cells are 

 suddenly differentiated and the borders of Retina A become indistin- 

 guishable. Retina A goes out of function when the tiny larva first bur- 

 rows into the mud, and the eye is blind until metamorphosis, when the 

 skin covering it becomes transparent and Retina B matures. No intra- 

 ocular muscles or suspensory-ligament fibers ever develop, for the pupil 

 is motionless and there is no ciliary body interposed between chorioid 

 and iris. The lens is propped in place only by the vitreous, which seems 

 to have evolved its semi-solid nature for this original purpose. 



Fishes : Except in the elasmobranchs, the optic vesicle is at first solid 

 as is the central nervous system, both eye and brain becoming hollow 

 secondarily. In many of the bony fishes the embryonic fissure never quite 

 closes, and the chorioid erupts through it to form the 'falciform process'. 

 Others develop, instead, a network of vessels at the vitreo-retinal inter- 

 face. Those species which have a pseudobranch develop a huge capillary 

 mass in the chorioid, the 'chorioid gland'. True lids and associated 

 glands are usually lacking, though vertical, so-called 'adipose' lids are 

 common. 



Amphibians: The fusion of the skin with the purely mesodermal, 

 inner layers of the cornea (those continuous with the sclera) is deferred 

 until metamorphosis, as is the development of the lids. The growing 

 suspensory-ligament fibers do not obliterate the anterior part of the 

 secondary vitreous, but remain embedded in it so that no aqueous-filled 

 cavities are ever formed behind the iris. Despite their entanglement, the 

 tertiary vitreous fibers are derived only from the ciliary epithelium, the 

 secondary vitreous solely from the sensory retina, just as in other verte- 

 brates. 



Reptiles and Birds: The neuroglial supporting tissue of the head of 

 the optic nerve usually proliferates a vascular, pigmented 'pecten' pro- 

 jecting through the vitreous toward the lens. In birds the elongated 

 base of the pecten follows the course of the embryonic fissure, developing 

 from its lips. In some groups, an anterior portion of the embryonic 

 fissure never closes, and a meridional slit is thus left in the ventral part 

 of the ciliary body, through which the anterior and posterior chambers 

 communicate. The third eyelid or nictitating membrane (present also, 

 as the 'haw', in many mammals) arises as a vertical fold of conjunctiva 

 at the nasal side of the eye, covered by the upper and lower lids. The 

 equator of the lens and the ciliary body come into contact and remain 



