274 JAMES EOLLIN SLONAKER 



ciliary bodies (fig. 41). The iris has grown only shghtly 

 farther over the lens but has been greatly thickened by increase 

 in the number of mesenchymatous cells. Numerous blood- 

 vessels are seen in it. The pigment of the retina, which has 

 become closely packed together and thinner, extends almost to 

 the free margin. The retinal portion covers the entire posterior 

 surface as a very thin membrane. There is no evidence of the 

 formation of muscles in the iris at this age. The retina and 

 pigment layers near the peripheral margin of the iris have become 

 much thickened and folded, so that the separation from the 

 developing chorioid is scarcely discernible. In fact, these three 

 structures seem to be more or less blended with each other in 

 forming the ciliary bodies (C6) . This is due to the fact that the 

 retinal layer, the pigment layer, and the mesenchyme each 

 takes part in the formation of the ciliary bodies. The aqueous 

 chamber is still limited in its extent to the region of the iris. 

 Nothing indicative of the ciliary muscles can be noted at this age. 

 The cells of all these structures still lack complete differentiation. 

 The early formation of the scleral plates is indicated by a denser 

 grouping of the cells in the more or less differentiated sclerotic 

 coat. 



By the thirteenth day (fig. 42) the cornea is beginning to 

 separate from the lens, so that the aqueous chamber now extends 

 completely across the lens as a thin space. This is apparently 

 due to an increase in the curvature of the cornea. The ciliary 

 body has become more definitely formed and the iris more typical 

 of the adult. 



Two days after hatching, all these structures are almost as 

 perfectly formed as in the adult. The tissue, however, are not 

 fully differentiated. The ciHary bodies are well formed. The 

 ciliary muscles are partly developed and some few cells show 

 shght striations. In the adult these muscles are all striated in 

 the bird. The scleral plates are formed. The iris is similar to 

 that of the adult, except that the muscles are not completely 

 striated. In the adult they are all striated. The thin layer of 

 retina back of the pigment layer of the iris is so covered and 

 impregnated with pigment that it is not visible. It can, however, 



