ORGANS OF SPECIAL SENSE 279 



seventh days the mesenchyme surroimding 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, w^hich 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 retinae after 

 the peripheral extension of the anterior chamber. It becornes 

 very vascular at an early stage. The canal of Schlemm arises 

 as a series 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 ligamentiim pectinatum iridis. 



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 



