THE NICTITATING MEMBRANE. 259 



The most important fact in this remarkable process of 

 eye-development is the circumstance that the optic nerve, 

 the retina, and the pigment-membrane originate from a 

 part of the brain, from a protuberance of the twixt-brain, 

 while the crystalline lens, the most important refracting 

 medium, develops from the outer skin (epidermis). From 

 the outer skin the horny lamina originates also the 

 delicate connecting membrane (conjunctiva) which after- 

 wards envelopes the outer surface of the eyeball The tear- 

 glands proceed, as branched processes, from the conjunctiva 

 (Fig. 214, p. 202). All the other parts of the eye originate 

 from the skin-fibrous layer; the vitreous body and the 

 vascular lens-capsule from the leather-plate, the choroid 

 coat with the iris, and the protective membrane (sclerotica) 

 with the horny membrane (cornea) from the head-plates. 



The outer protective organs for the eye, the eyelids, are 

 merely simple folds of skin, which, in the human embryo, 

 appear in the third month. In the fourth month the upper 

 eyelid adheres to the lower, and the eye then remains 

 covered by them till birth. (Plate VII. Fig. M ill., R in., 

 etc.) The two eyelids usually again separate shortly before 

 birth, but sometimes not till after. Our skulled ancestors 

 had, in addition to these, a third eyelid, the nictitating 

 membrane, which was drawn over the eye from the inner 

 corner. Many Primitive Fishes (Selachii) and Amnion 

 Animals yet retain this. In Apes and in Man it has atrophied, 

 and only a small remnant of it exists in the inner corner of 

 the eye as the " crescent-shaped fold," as a useless " rudi- 

 rnontary organ." (Cf. voL i. p. 109.) Apes and Man have 

 also lost the " Harder gland," opening below the nictitating 

 membrane, which appears in other Mammals, and in Birds, 

 Reptiles, and Amphibians. 



