180 



ESSENTIALS OF ZOOLOGY 



forebrain, and in the development of accessory structures. The 

 cup comes into contact with the skin and is folded inwards. 

 The skin ectoderm opposite the cup thickens and forms the 

 lens. The cup by growth and a continuation of the involu- 

 tion is converted into two layers closely adposed, with the 

 opening directed outwards. The inner forms the retina and 

 the outer the pigment layer of the retina. The cup and the 

 lens are enclosed in a double capsule derived from mesoderm. 

 The inner is the choroid coat, which, like the retina, ends exter- 

 nally in a margin. The space denned by the margin is called 

 the pupil. The margin forms the iris, and dorsally it is 



Forebrain 



Optic cup Stalk Lens 



Lens Pigment Retina 

 layer 



Retina Lens 



Conjunctival 

 ectoderm 



FIG. 90. Torpedo. Development of eye. After Froriep. 



folded to form a series of processes. The sclerotic is carti- 

 laginous and completely surrounds the other structures. 

 Opposite the iris and lens it is transparent, and this part of 

 the sclerotic is termed the cornea. It is covered by a thin 

 transparent conjunctiva continuous with the skin. The 

 mesoderm enclosed by the retinal cup is converted into a 

 transparent vitreous humour of gelatinous consistency, and the 

 space between the lens and the cornea is filled with a watery 

 fluid called the aqueous humour. The retina develops the 

 sensory receptor rods and cones and nerve cells and fibres 

 associated with them. The fibres are assembled to form the 

 optic nerve, which leaves the eye through the stalk which con- 

 nects the retinal cup with the thalamencephalon. The nerve 

 therefore pierces the choroid and the sclerotic coats. The 

 movements of the eye in the orbit are produced by the muscles 

 mentioned above, and the eye is supported on a cartilaginous 



