288 JAMES ROLLIN SLONAKER 



THE CHORIOID AND SCLEROTIC COATS 



At the age of two days' incubation the sparrow eye shows a 

 condensation of the mesoderm just outside of the pigment layer 

 (figs. 48 and 49). Most of these cells are undifferentiated. 

 Immediately in contact with the pigment layer are a number 

 of modified, spindle-shaped cells, with large nuclei, their long 

 axes parallel with the pigment layer. A row of these cells, one 

 cell deep, is rather definitely arranged next to the pigment layer 

 and extends almost to the periphery of the pigment layer. Some 

 blood-vessels occur just outside of and in close relation to this 

 layer. This row of cells and the adjacent blood-vessels constitute 

 the developing chorioid. 



Other spindle-shaped mesodermal cells are arranged parallel 

 to and just outside the blood-vessels of the chorioid. They are 

 most pronounced and differentiated at the optic axis, and they 

 grow rapidly less distinct toward the periphery. They wholly 

 disappear at about one-third the distance to the margin of the 

 optic cup. The innermost portion of these cells will form later 

 the outer boundary of the chorioid coat; the remaining cells 

 represent the early stage in the developing sclerotic coat. 



With three days of incubation the cells of the chorioid have 

 become more numerous, especially in the region of the axis 

 (figs. 50 and 51), and, with the adjacent blood-vessels, form a 

 more definite layer. The formative cells of the sclerotic coat 

 can now be traced fairly well to the lens. The cells are more 

 definite in outhne This is especially true at the axis where they 

 are most numerous. 



By the fourth day the cells constituting the chorioid and 

 sclerotic coats have become consp'cuous and rather sharply 

 marked off from the surrounding loose mesodermic structure 

 (fig. 52). The combined measurement of these two coats at 

 the axis is 0.08 mm. and at the equator 0.0024 mm. One day 

 later, the maximum thickness at the axis is 0.108 mm. (figs. 53 

 and 54). 



The part which these layers take in the formation of the cornea, 

 iris, ciliary bodies, etc., has already been described under the 

 heading, the cornea, iris, and aqueous chamber. 



