DEVELPOMENT OF EYE OF SPARROW 307 



able thickening are the nerve fiber, the ganglion cell, and the 

 inner molecular (table 4). At the ora serrata the retina has 

 begun to thin, largely due to a reduction in thickness of the 

 inner nuclear layer (table 5). The ganglion-cell layer is three 

 and occasionally four cells deep. This is true throughout its 

 extent. A noticeable difference is observed in the arrangement 

 of these cells in different parts of the retina. In the axial center 

 they are very compact, but in the periphery they are loosely 

 arranged, though still conforming to a row three cells deep. The 

 inner nuclear layer is about three cells deep, and the cells show 

 about the same general arrangement throughout its extent. 

 The five divisions of the inner molecular layer described by 

 Ramon y Cajal can be distinguished. 



The pigment layer is now very densely pigmented, but the 

 granules are wholly within the cell bodies. Tangential sections 

 through these cells show that the granules are located largely 

 in the peripheral portions of the cell, leaving the nuclear region 

 almost free. 



A continued decrease in the total thickness of the retina is 

 observed in the young sparrow four days after hatching. This 

 is due, in my opinion, to a spreading of the retinal cells over a 

 wider area to keep pace with the rapid increase in the size of 

 the eye. The great enlargement in the surface of the retina is 

 shown in table 6. So far as I have been able to observe, there 

 is no multiphcation of the cells of the retina after hatching. 

 There is, however, an increase in the size of some of them, espe- 

 cially the ganglion cells. Evidence indicates that extension 

 of the retina in a peripheral direction, due to growth of the eye, 

 is accomplished by a migration of the retinal cells in this direction. 

 The reduction of the ganglion cells to a layer one cell deep, and 

 a lack of continuity of these cells, as well as a reduction in number 

 of cells of the other nuclear layers, point conclusively to a migra- 

 tion unaccompanied by an increase in the number of cells. 



At this age the pigment cells show that a very shght move- 

 ment of pigment granules toward the rod-and-cone layer has 

 occurred. 



