294 



LEISURE-TIME STUDIES. 



the outer world, and thus comes as the nervous system to 

 regulate and control the entire organism. 



But the process of formation of the nervous system from 

 an infolded layer of the original outer surface of the body, is 

 equally well seen in the development of the eye, ear, or 

 nose, those specialised parts of the nervous system through 

 which we obtain a defined knowledge of the world around 

 us. On the second and third day in the development of 

 the chick, the formation of the eye and ear proceeds apace. 

 Both organs are formed by an infolding of the outer or skin- 



FIG. 57. Development of the eye. 



layer, the epiblast ; this fold growing inwards to meet and to 

 unite with an outgrowth from the brain. In the accompany- 

 ing figures (Figs. 57 and 58) the formation of the ear and eye 



FIG. 58. Development of the ear. 



is illustrated. At ;/, in the illustration of the development of 

 the eye, the outgrowth from the brain is figured, and at / the 

 ingrowth of the skin-layer, which becomes the lens (/), with 

 the aqueous humour (B, /) in front. The brain-outgrowth 

 (;/) ultimately forms the retina (c, n), and is connected to the 

 brain by the optic nerve (op). The " vitreous humour " of 

 the eye is shown at z>, as growing in between the lens and 

 the brain-outgrowth. The eyelids are formed by two skin- 

 folds (c c). The ear is developed from a sac or folded-in 

 portion of the skin-layer (/), which itself forms what is 



