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 
erow towards the center, and thus a cavity, the conjunctival 
sac, is formed between the folds and the integument over the eye- 
ball (conjunctiva sclerz). 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. 
Papille Conjunctiva Sclere. On the seventh day of incubation 
papille begin to appear on the surface of the conjunctiva sclerz 
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 
papille 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 papille 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). 
