340 ZOOLOGY FOR MEDICAL STUDENTS chap. 



outer side is in close contact with the external epidermis which is con- 

 tinued over it as a thin transparent layer — the conjunctiva (Fig. 141, c). 



A protective flap of skin grows partially over the eyeball from below 

 and another from above. These are the eyelids. 



The eyeball is connected with the brain by the thick optic nerve 

 (Fig. 141, o.n) formed by nerve-fibres which converge from the inner 

 surface of the retina towards a point near its centre to form a solid 

 nerve-trunk which passes straight through the wall of the eyeball and 

 is continued on through the orbit towards the brain. 



Development of the Eye. — One of the most fascinating chapters 

 in vertebrate embryology is that dealing with the development of the 

 eye. Any student possessed of an elementary acquaintance with the 

 methods of cutting sections can follow out the main steps in the process 

 for himself on material obtained from hen's eggs which have been 

 incubated from about i^ days onwards. On this account the description 

 here will deal with the eye of the Bird which, however, agrees in all its 

 main features with that of the Dogfish. 



The retina with its stalk the optic nerve is simply a projecting and 

 specialized portion of the wall of the brain. It will" be recalled that the 

 brain is the dilated anterior portion of the neural tube, and that its 

 cavity is a portion of the outer world which has been enclosed in the 

 formation of the tube : the inner surface of its wall is part of the original 

 outer surface of the body which has been tucked inwards. 



Inspection of a fowl embryo from an egg which has been incubated 

 about a day and a half (see Ifig. 192, p. 459) shows that the brain has 

 assumed a T-shape — the fore-brain projecting on each side so as to be 

 in contact with the outer skin. The projection mentioned is the optic 

 rudiment. The connexion of this with the central part of the brain 

 (thalamencephalon) becomes relatively narrowed and is now known 

 as the optic stalk. The outer end of the rudiment becomes gradually 

 tucked into its interior so that the rudiment takes the form of a 

 double-walled optic cup (Fig. 142, B and C). The tucking-in process 

 is not confined to the outer end of the rudiment but is continued along 

 its ventral side on to the optic stalk. The cup therefore is not complete 

 but has a gap along its ventral wall — the choroid fissure, and the optic 

 stalk is no longer round in section but (ft -shaped. Of the two layers of 

 the optic cup wall the inner (r) gradually thickens and undergoes com- 

 plicated changes in detail and becomes the functional retina. It is now 

 apparent why the rods in the fully developed retina point away from 

 the lens, for it is this surface of the retina which is next the enclosed 

 portion of the outer world that forms the cavity of the brain : in other 



