THE VERTEBRATE EYE 241 



formed by a single ectodermal layer ; but into the retina of Vertebrates 

 is thus aggregated the analogue of the ojitic ganglion of Invertebrates ; 

 it becomes an island of the central nervous system, and the optic nerve 

 becomes a tract of this system connecting the outlying part with the 

 main body. 



In the vast majority of cases we have seen that the receptor end 

 of the sensory cell in the epithelial eye of the Invertebrate lay towards 

 the surface of the body/ but when it was enfolded in the neural tube 

 of the Vertebrate, this end now lay deeply and the pole from which the 

 nerve fibre issues became superficial (Fig. 247). It follows that in the 

 cerebral eye of the Vertebrate, light must traverse the whole thickness 

 of the retina in order to reach the sentient layer ; such an arrangement 

 we have already called an inverted retina in contradistinction to the 

 more primitive verted retina wherein light first strikes the visual 

 cells before reaching their nervous prolongations.^ The inverted retina 

 may seem an anomalous arrangement from an optical point of view, 

 but it carries the advantage that the visual receptors can be brought 

 into contact with the pigment and that the part of the retina in which 

 the greatest activity occurs lies nearest the caj)illaries of the choroid ; 

 both of these — pigment and a dense layer of blood-vessels^ — for optical 

 reasons could only be situated deeply to the visual elements. More- 

 over, an inverted arrangement allows the evolution of intracellular 

 colour filters within the visual cells (Walls and Judd, 1933) and permits 

 an increase of the resolving power of the central region by the formation 

 of a fovea (Walls, 1937). 



The remainder of the eye is derived from the surrounding ecto- 

 dermal and mesodermal tissues. The surface ectoderm devotes itself 

 entirely to the formation of the dioptric apjDaratus. an arrangement 

 which allows greater efficiency than was the case in Invertebrates in 

 which a refringent mechanism was developed from the same layer as the 

 sentient cells themselves. Intercalary cells in the sentient layer, 

 however, retain this function to some extent by secreting a transparent 

 medium (the vitreous). Organs of protection are provided from the 

 surrounding mesodermal tissues — a fibrous sclerotic coat, lids, a 

 lacrimal apparatus, and a bony orbit ; and from the same source a 

 motor apparatus is added, and a vascular system provided. 



Franz. Bolk's Hb. d. vergl. Anat. d. Lange. Zbl. prnkf. Angeuheilk., 32, 131 



Wirbeltiere, Berlin, 2 (ii), 989 (1934). (1908). 



Froriep. Hb. d. vergl. u. e.rper. Entwick- Stone. Anat. Fee, 106, 89 (1950). 



lungslehre d. Wirbeltiere, Jena, 2 Walls. Arch. Ophthal. (Chicago), 18, 912 



(1906). (1937). 



.4no?. ^nz. (Snppl.), 29, 145 (1906). Walls and Judd. Brit. J. Ophthal., 17, 



Kerr, Graham. Te.rtbook of Embryology. 641, 705 (1933). 



London (1919). 



1 p. 146, 2 p. 146. 



S.O.— VOL. I. IG 



