DEVELOPMENT OF EYE OF SPARROW 289 



At the age of seven days (fig. 56, pi. 8, and pi. 9) the blood- 

 vessels of the chorioid are more definite and form a fairly con- 

 tinuous open network, 0.004 mm. thick at the equator and 

 0.008 mm. at the axis. Its outside boundary is irregular, due 

 to a lack of uniformity of the scleral border. The parallel cells 

 of the sclerotic coat now form a fairly definite layer which meas- 

 ures 0.020 mm. at the equator and 0.028 mm. at the axis. No 

 differentiation of these cells into cartilage and fibrous portions 

 can be discerned. Neither is there any pigment in the cells of 

 the chorioid. 



In the nine-day embryo (fig. 69) the chorioid shows little 

 advancement, except an increase in the vascular part. The 

 sclera, however, shows marked differentiation. A dense mass 

 just outside the chorioid now appears as embryonic cartilage. 

 This cartilage-like mass of cells is thicker in the axial region 

 and thinnest at its margins near the ora serrata. On either 

 side of these cartilage-forming cells are spindle-shaped cells 

 which will later form the remaining portion of the sclerotic coat. 

 At the anterior end of the cartilage-hke mass the cells are more 

 closely arranged in an elongated slender portion which is a very 

 early stage in the formation of the scleral, or bony plates. 



Little change is noticed in the chorioid of the ten-day embryo 

 (figs. 70 and 71). The blood-vessels are much more abundant 

 near the pigment layer of the retina than in the outer portions 

 of the chorioid. The cartilage portion of the sclerotic coat is 

 more sharply marked off from the other part. The cells forming 

 the remaining part are packed closer together into an apparently 

 firm mass. The thickness of these two coats at different ages 

 described is shown in table 3. In the early ages these structures 

 were too indefinite to measure accurately; no measurements 

 are therefore tabulated for them. Although differentiation is 

 most pronounced in the axial region and decreases toward the 

 periphery, the table shows that these coats are thicker at the 

 ora serrata than at the axis at about this age. This is mainly 

 due to the closer arrangement of the cells in the axial region. 



About the age of hatching (fig. 75, F, and figs. 78 and 79, F) 

 the first modification of the chorioid, the precursor of the area 



