CHAPTER XVIII. 



THE ORGANS OF SPECIAL SENSE. 



THE EYE. 



The receptive mechanisms of all the general and special sense organs 

 are derived from the ectoderm. With the single exception of the eye, all 

 develop as direct specializations of the ectoderm in the form of the various 

 neuro-epithelia. The eye is peculiar among the sense organs in that its recep- 

 tive cells are not derived directly from surface ectoderm, but only indirectly from 

 the ectoderm after it has become folded in to form the neural canal. The 

 neuro-epithelium of the eye develops as a direct outgrowth from the central 

 nervous system. The retina is a modified part of the brain; the optic nerves 

 correspond to central nervous system fiber tracts. Of the accessory optic 

 structures, the lens, the epithelium of the lids and conjunctiva, the eyelashes, 

 the Meibomian glands and the epithelium of the lacrymal apparatus are of 

 ectodermic origin; the coats of the eye, the sclera and chorioid, and parts of 



Optic Neural 



depression plate 



Fig. 494. — Diagram showing location of optic areas before the closure of the neural groove. 



Modified from Lange. 



their modified anterior extensions, the cornea, ciliary body and iris, are of 

 mesodermic origin. In the sensory divisions of the other spinal and cranial 

 nerves, with the exception of the olfactory, the cell bodies of the neurones which 

 serve to connect the receptive mechanisms with the brain and cord are located 

 in parts (the sensory ganglia of the cranial and spinal nerves) which have be- 

 come separated from the crests of the neural folds as the latter fuse to form the 

 neural canal. In the eye the cell bodies of these neurones are located in the 

 retina, but the area of ectoderm from which the retina develops first occupies a 

 position along the neural crest analogous to that occupied by the anlagen of the 

 spinal and cranial ganglia. In the case of the retina this area, instead of be- 

 coming split off in the closure of the neural canal, becomes folded into the 

 canal and later pushed out toward the surface in the optic evagination (Figs. 494, 



495- 49 6 )- 



573 



