DEVELOPMENT OF EYE OF SPARROW 315 



A single row of similar cells occurs in the posterior portion of 

 the iris, between the pigment portion and the mass of tissue in 

 front of it. This row later forms the radial muscles of the iris. 

 The cells which lie in front, and which will later form the circular, 

 or sphincter muscles of the iris, appear as rounded nuclear bodies. 

 This is due to the plane of section cutting these cells at right 

 angles to their long axes. The different views as to the origin of 

 these muscles are discussed in dealing with the development of the 

 cornea, iris, and aqueous chamber. 



Two daj^s after hatching these structures have become more 

 typical of muscle tissue. The cells have become greatly 

 elongated, the mass more definite in shape, and a few faint 

 striations can be made out in the muscle cells. Jiy the fourth 

 day these cross-striations have become more pronounced and 

 can be easily observed. Development proceeds rapidly, so that 

 by the time the eyes open these muscles are fully striated and 

 ready to function. 



THE EYELIDS 



The first appearance of any of the structures of the lids is 

 seen in the chick at the age of about sixty-four hours' incubation, 

 just after the complete formation of the lens vesicle. The 

 ectoderm, now covering the lens, becomes later the epithelium 

 of the conjunctiva (figs. 45, 46, and 47). At this age the ectoderm 

 covering the head and eyes shows no differentiation. In the 

 chick, even at the age of 100 hours, this layer is still uniform 

 over the surface. 



The earliest age of the sparrow (about two days) which I 

 have secured corresponds fairly closely to the four-and-one-half- 

 day chick, and is slightly further advanced than that of the 

 100-hour chick shown in figure 47. The sparrow at the age of 

 two days is shown in figures 1, 17, and 48. 



The first indication of the lids is seen in the four-day sparrow 

 embryo (figs. 20 and 54, A^. In the five-day sparrow embryo 

 it is more pronounced. It consists of a slight swelling on the 

 surface at the anterior margin of the eyeball, which is due to an 

 increase, or outgrowth, of mesodermal cells and not to any 



