ORGANS OF SPECIAL SENSE 279 
seventh days the mesenchyme surrounding the eyeball begins 
to penetrate it from all sides in the form of a compact wedge, 
which, advancing in the substance of the cornea propria, soon 
meets in the center. These cells form the so-called corpuscles 
of the cornea. They appear arranged in strata from a very 
early period. 
The anterior chamber is bounded by the cornea externally; 
its margins, which are at first coincident with the lips of the optic 
cup, soon extend peripherally over the iris (Fig. 159). The inner 
epithelium ceases at the margin of the cavity or is continuous 
with the cells of the sclerotic; it does not appear, in an eight-day 
chick at any rate, to be reflected over the iris, but the epithelium 
of this structure next the anterior chamber appears to be simply 
a special differentiation of its own superficial cells. The anterior 
chamber is closed centrally by the lens, but communicates more 
or less for a considerable period around its margin with the pos- 
terior chamber. This is at least the appearance in good sections; 
it seems probable, though, that in life there is contact between 
the optic cup and lens. 
The stroma of the iris proceeds from that portion of the 
mesenchyme left in association with the pars iridis retine after 
the peripheral extension of the anterior chamber. It becomes 
very vascular at an early stage. The canal of Schlemm arises 
as aseries of vacuoles just peripheral to the margin of the ante- 
rior chamber about the eighth day. These soon run together 
to form a ring, which is separated from the anterior chamber 
by the ligamentum pectinatum tridis. 
5. The choroid and sclerotic coats are differentiations of the 
mesenchyme surrounding the optic cup. But little is known 
concerning the details of their development in the chick. A 
figure of Kessler’s shows chromatophores developed in the choroid 
coat at twelve days; I find a very few already formed at eight 
days. Cartilage begins to appear in the sclerotic at eight days, 
the forerunner of the sclerotic ossicles (Fig. 159). 
6. The Eyelids and Conjunctival Sac. The integument over 
the embryonic eyeball remains unmodified until about the 
seventh day. At this time a circular fold of the integument 
forms around the eyeball with the pupil as its center. At the 
same time a semi-lunar fold develops within the first on the side 
of the eyeball next the beak. (See Figs. 122-124.) From the 
