280 THE DEVELOPMENT OF THE CHICK 



first fold the upper and lower eyelids are developed, and from the 

 second the third eyelid or nictitating membrane. The area bounded 

 by the outer ring-shaped fold becomes the conjunctival sac. 



From their place of origin the free edges of these folds then 

 grow towards the center, and thus a cavity, the conjunctival 

 sac, is formed between the folds and the integument over the eye- 

 ball (conjunctiva sclerse). The outer fold grows more rapidly 

 above and below than at the sides and the opening narrows, 

 becoming, therefore, gradually elliptical and finally somewhat 

 spindle-shaped. Thus the upper and lower eyelids are established. 

 The semi-lunar fold of the embryonic nictitating membrane also 

 grows towards the pupil, most rapidly in its center. The con- 

 junctival sac also expands peripherally, especially at the inner 

 angle of the eye, and thus accommodates itself to the increasing 

 size of the eyeball (Fig. 159). 



The Harderian gland is visible on the eighth day as a solid 

 ingrowth of ectodermal cells of the conjunctival sac at the inner- 

 most angle of the nictitating membrane. 



Feather germs develop on the outer surface of both upper 

 and lower lids especially at their edges. The ectoderm covering 

 the inner faces of the upper and lower lids, both faces of the nic- 

 titating membrane and the remainder of the conjunctival sac 

 becomes modified into a moist mucous membrane. Over the 

 cornea the ectoderm is especially modified as already noted. 



Papillce Conjunctiva Sclerw. On the seventh day of incubation 

 papilla? begin to appear on the surface of the conjunctiva sclerse 

 and soon form a ring surrounding the iris at some distance periph- 

 eral to its margin (Figs. 122, 123 and 124). The number of these 

 papilla? appears to be quite constantly fourteen. They are at first 

 fully exposed owing to the undeveloped condition of the eyelids, 

 but the latter overgrow them about the eleventh or twelfth days. 

 Degeneration of the papilla? begins about this time, and on the 

 thirteenth day they have entirely disappeared. In section they 

 are found to be thickenings of the ectoderm, produced by multi- 

 plication of the cells. They may rise above the surface; but more 

 frequently project inwards towards the connective tissue. There 

 is apparently no accompanying hypertrophy of the latter. Thus 

 they differ quite essentially from feather germs with which it 

 seems natural to compare them; and their significance is entirely 

 problematical (see Nussbaum). 



