IV. VERTEBRATA. 541 



similar ceplialopod ej-e (p. 'iSo), tlie most striking difference re- 

 mains to be mentioned. Tlie retina, "with its liniitans interna and. 

 nerve-iibre layer, abuts against the Titreons body; witli its rhab- 

 donies and tapetum against tlie choroid. Hence tlie incoming 

 light mnst trav'erse the optic ganglion and pass through the layer of 

 sense cells before reaching the end organs, the rhabdomes. In 

 nearly all invertebrates, for example the Cephalopoda (fig. 383), 

 tha light falls directly on tlie peripheral end of the rhabdome. 

 The rhabdomes in cephalopoda, as in most invertebrates, are 

 turned towards the light, in the vertebrates away from it. 



This pecuhar and functionally purposeless inversion of the vertebrate 

 retina is explained by the development of the eye. This can be divided, 

 according to origin, into two parts, a cerebral (optic nerve, retina, tape- 

 tum) and a peripheral (all other parts). As the eye in tunicates and Am- 

 2M0XUS is permanently a part of the brain, so is the retina of vertebrates 

 genetically, and of the first cerebral vesicle. An outgrowth occurs on 

 either side (fig. 574, B) of the 'twixt brain and becomes expanded distally 



Fig 574 -Diagram showinc the inversion of layers in the formation of the retina 

 (oris ) The nuclei are placed in the (morphologically) deeper ends of the cells. 

 In A the brain (h) has been closed in ; in S the optic vesicle (rt ha,s reached the 

 lens (!) and on the right is being converted into the double-walled optic cup 

 with, as shown in C, an outer tapetal (e) and an inner retinal layer (r). 



to an optic vesicle which is connected with the brain by an optic stalk. 

 The vesicle extends out to the periphery and, coincidently with the de- 

 velopment of the lens, is folded into a double-walled optic cup with outer 

 or tapetal, inner or retinal layers. If the position of the epithelial cells 

 be followed, it will be seen that the peripheral ends rest upon the tapetum, 

 and when these ends develop the rhabdomes, these must grow into the 



tapetal layer. 



In contrast to the retina, the lens develops as an invagination from 

 the epithelium of the body (flg. 574) ; sclera, cornea and vitreous bo<ly from 

 connective tissue. Thus the important part of the eye arises from the 

 brain and is later provided with accessory apparatus which arise from 

 peripheral parts. The invertebrate eye, on the other hand, with all its 

 parts arises from the skin. 



The vertebrate eye is furnished with secondary structures : with mus- 

 cles which move it, with lids which protect the cornea from injury and 

 drying The lids are dermal folds which extend over the eyeball from 

 above and below. To these a third lid, the nictitating membrane, may 



