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). 



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

 and another from above. These are the eyelids. 



Tin- eyeball is connected with the brain by the thick optic nerve 



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 riuitinued 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 

 Any student possessed of an elementary acquaintance with the 

 in ct hods of cutting sections can follow out the main steps in the process 

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

 in* -abated from about li days onwards. On this account the description 

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

 main features with that of the Dogfish. 



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

 >pcciali/ed 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 Fig. 192, p. 459) shows that the brain has 

 a.-vsmned a T -shape the fore-brain projecting on each side so as to be 

 in conta-t 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 

 a^ 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 

 ' ntral side on to the optic stalk. The cup therefore is not complete 

 I uit ha- a -ap along its ventral wall the choroid fissure, and the optic 

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

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

 banges in detail and becomes the functional retina. It is now 

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

 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 





