THE LATER DEVELOPMENT OF THE CHICK 321 



irregularly, but finally they assume a definite and regular 

 arrangement. Layers are formed during the entire growth 

 period of the organism and the number finally laid down is 

 very large. At first the lens is in contact with the surface 

 ectoderm, but it soon moves within the pupil, leaving a space 

 between itself and the ectoderm. This is the beginning of 

 the anterior chamber of the eye (Fig. 127, D); it extends beyond 

 the margin of the iris, and soon becomes invaded by mesen- 

 chyme cells, part of which are added to the iris, while others 

 form the larger part of the cornea. 



The cornea is only to a slight degree formed by the surface 

 ectoderm without the Ms and pupil; only its superficial epi- 

 thelium is thus derived. The major portion of the cornea is 

 derived from mesenchyme cells which first form a single layer 

 within the ectodermal epithelium (Fig. 127, D), and then later 

 enter in large numbers between these two epithelia, forming- 

 layers of corneal cells. From the mesenchyme surrounding 

 the entire optic cup are derived the choroid and sclerotic coats 

 of the eye-ball. 



2. The Ear 



Like the eye, the fully formed ear is a complex, formed of 

 elements of diverse origin, which become morphologically and 

 functionally associated during development. The ear develops 

 somewhat later than the eye, in the region of the myelen- 

 cephalon. Making the customary distinction between the 

 primary or sensory, and the secondary or accessory portions, 

 we see that the true sensory portion is formed from the surface 

 of the head, rather than the wall of the nervous system. The 

 accessory parts are derived from the hyomandibular visceral 

 pouch and the mesenchyme of the region. 



The first indication of the ear appears about the thirtieth 

 hour as a circular thickening in the ectoderm on the side of the 

 head, just in front of the level of the first mesodermal somite. 

 This patch enlarges, becomes depressed and invaginated, and 

 about the beginning of the third day it has formed a consider- 



