578 



VERTEBRATE LIFE AND ORGANIZATION 



Neural -Bold 



Optic ve-sicle 



Surfa.ce 

 ctoderm- 



Lens 

 placode- 



Developin^ brain- 



B 



Lens 

 vesicle 



Surfa.ce — ' 

 ectoderm 



—Pigment layer of retina. 

 pNervous laye-r of retina 



Cornea- 



I— Brain 



ectoderm 



■ Optic stalK 

 ^ Optic cup 



C 



Sclera 



Choroid 

 Retina 



D 



Figure 29.3. The development of the eye. A, Cross section through an embryo 

 in which the anterior portions of the neural folds are closing to form the brain; 

 B, the optic vesicles evaginate from the sides of the forebrain; C, an optic cup develops 

 from each optic vesicle and the lens forms from adjacent surface ectoderm; D, the 

 choroid, sclera and part of the cornea develop from surrounding mesoderm. Arrows 

 indicate the original polarity of the ectoderm cells. (D, From Romer.) 



neuron chain that extends to the brain; a number of rods, on the other 

 hand, usually converge upon a single neuron (Fig. 29.2 B). Thus, 

 light that falls upon the cones is translated into a sharper image in 

 the brain than that falling upon the rods. Cones are concentrated 

 near the center of the retina, and are particularly abundant in an 

 area known as the fovea, which is the region of keenest vision in 

 bright light. However, if one wishes to see in dim light one must 

 look somewhat to the side so the image of the object will fall on 

 the periphery of the retina where there are more rods. Neither rods 

 nor cones are present in the part of the retina through which the 

 optic nerve passes, hence this region is called the blind spot. 



The cavities within the eye are filled with liquid. A gelatinous 

 vitreous humor occupies the large chamber that lies between the 

 lens and the retina, and helps to hold the lens in place. A watery 

 aqueous humor fills the posterior chamber between the iris and 

 the lens, and the anterior chamber between the iris and the cornea. 

 The aqueous humor is secreted continually by the ciliary body and 

 drained through the canal of Schlemm at the base of the cornea. 

 By maintaining the intraocular pressure, the aqueous humor helps 

 to maintain the turgidity and shape of the eyeball. Blockage of the 



