316 JAMES EOLLIN SLONAKER 



change in the ectoderm. This is the beginning of the nictitating 

 membrane, or third eyeUd. This elevation increases in size, and 

 at the same time begins to grow backward over the eye. At the 

 sixth day a second swelhng appears just anterior to that of the 

 nictitating membrane (fig. 21), and a similar thickening occurs 

 at the opposite side of the eye. These swelHngs are the begin- 

 nings of the true lids. Up to this age the full size of the eyeball 

 is very conspicuous, as it is covered only by the thin ectoderm. 

 By the seventh day these elevations have become quite promi- 

 nent, and those forming the upper and lower lids have extended 

 completely around the eye (figs. 22 and 56). They seem to be 

 growing out over the eye, but this appearance is due more to the 

 increasing size of the eyeball than to an encroachment of the 

 forming lids. 



This rapid growth of the lids and the eyeball continues, so 

 that by the ninth day the nictitating membrane has grown 

 backward almost to its full extent (fig. 26 and 69). The ecto- 

 derm, which covers the cornea, completely encloses the nicti- 

 tating membrane, and lines the ocular side of the lids, has become 

 more epithelial-like in character. At the extreme anterior 

 fornix, where it is reflected from the cornea over the nictitating 

 membrane and finally from the anterior surface of this membrane 

 to the lining of the Uds, it presents a uniform and smooth surface. 

 At the free margin of the lids the ectoderm is beginning to 

 thicken slightly. 



At this age the first slight elevations of the plumules of the 

 lids and feathers of the head begin to appear. They are first 

 developed on the upper part of the head and the upper lid. The 

 interpalpebral space is still quite large. The ratio of the diam- 

 eter of this space to that of the eyeball is 9:11. 



The nictitating membrane has become somewhat thinner, 

 but in cross-section is still decidedly club-shaped, bending 

 slightly to conform to the surface of the cornea. The thickest 

 portion is toward the free margin and the thinnest at the attach- 

 ment at the fornix. The thickness at these two regions is 0.112 

 mm. and 0.040 mm., respectively. The cells constituting the 

 central portion of the nictitating membrane are widely separated, 

 as compared with the corresponding portion of the true lids. 



