DEVELOPMENT OF EYE OF SPARROW 317 



The appearance suggests that the increase in the number of 

 mesodermal cells has not kept pace with the growth in size of 

 this structure and that the latter is due to an inflow or secretion, of 

 a clear intercellular fluid. Blood-vessels extend almost to the 

 free margin. 



At twelve days' incubation (fig. 75) the nictitating membrane 

 is beginning to differentiate into the marginal plate and a much 

 thinner proximal portion, somewhat like the adult. The conjunc- 

 tival epithelium is thicker in the axial part of the cornea than 

 elsewhere. Here it is composed almost wholly of columnar 

 cells with about one thin layer of the stratified type at the outer 

 margin. From this region it diminishes gradually, from 0.024 

 mm. in thickness, to the fornix, where it measures 0.012 mm. 

 It maintains about this thickness over the nictitating membrane 

 and the ocular surface of the lids. Throughout its entire extent 

 it forms a smooth covering on these surfaces. At a later period 

 of development the conjunctiva is thrown into numerous folds 

 at the fornix conjunctivae. This arrangement allows free 

 movement of the lids and eyeball without injury to the epithelium. 



By the thirteenth day, when the bird hatches, all of the lids 

 have reached practically the adult structure and form (fig. 30). 

 The nictitating membrane has become thin and the marginal 

 plate is well developed and stiffened by a cartilage-like plate. 

 This doubtless corresponds to the plate of hyaline cartilage 

 which Schafer ('97) says is found in the nictitating membrane 

 of many adult animals. The marginal folds of the lids are con- 

 spicuous. The plumule papillae are well developed, but the 

 plumules do not appear until some time after hatching. The 

 ratio of the horizontal diameter of the interpalpebral space to 

 that of the eye is as 13:40. This is practically the same as that 

 of the adult, which is 1:2. The ratio in the vertical direction 

 cannot be estimated, as the lids are partly closed. 



At two days after hatching, folding of the epitheUum at the 

 fornix conjunctivae has begun. This is shown in figures 97, 98, 

 99, 100, and 102. The greatest folding of the conjunctiva 

 occurs on the base of the nictitating membrane. The structure 

 of the lids is now practically the same as has been described for 

 the adult (Slonaker, '18). 



