DEVELOPMENT OF EYE OF SPARROW 311 



conclusions. I have found that the retma is thicker at the 

 optical axis than elsewhere, and that differentiation into layers 

 begins at that point. This thickening and differentiation is 

 gradual and general, and hardly characteristic of the true area 

 centraUs which occupies a rather definite region, which is first 

 indicated by a special modification of the chorioid at the optical 

 axis. 



The first indication of the developing area and fovea centralis 

 which I have found in the sparrow antecedes that described by 

 Chievitz. It is not a local thickening of the retina, but a special 

 thickening of the chorioid coat, immediately back of the region 

 where the fovea later develops. This is not at first accompanied 

 by any observable change in the retina. The special thickening 

 of the chorioid is entirely distinct and easily distinguishable 

 from the general increase in thickness of the chorioid in the axial 

 region as compared with the region of the ora serrata. 



As early as the tenth day of incubation the chorioid coat shows 

 a marked increase in thickness from the periphery to the axial 

 region where it is thickest. This, however, is a gradual augmen- 

 tation and not the pronounced and special increase which deter- 

 mines the location and formation of the fovea. It is nevertheless 

 true that the region of maximum thickness is where the fovea 

 will develop. As shown in figure 70, there is no special modifi- 

 cation of the retina in this region at this age. The retina, how- 

 ever, has its greatest measurement at this point. 



At twelve to thirteen days' incubation, the age of hatching, 

 the chorioid measures about 0.008 mm. at the ora serrata and 

 gradually increases to 0.015 mm., in the central portion. In a 

 small region at the optical axis it rapidly increases to a maximum 

 thickness of 0.0200 mm. This increase, due to a greater number 

 of blood-vessels, marks the location of the area centralis and 

 later the fovea. 



Closely following this richer blood supply is a rapid increase 

 in the number of retinal cells accompanied by a marked thicken- 

 ing of the chorioid (fig. 86). By the fourth day after hatching 

 the condition of thickened retina is rapidly followed by a slight 

 thinning of the retinal cells in the optical axis due to a radial 

 migration from this point (fig. 91). 



