DEVELOPMENT OF EYE OF SPARROW 301 



In describing the differentiation of the retinal cells in the cat, 

 Ramon y Cajal ('96) says that the multipolar cells (ganglion 

 cells) are first to develop and are very soon followed by the growth 

 of axis-cylinder processes (nerve fibers), then by the dendrites. 

 He also says that at an early date the bipolar cells, which later 

 form the rods, can be distinguished from those which will form 

 the cones. Each sends processes toward the external limiting 

 membrane and, later, one in the opposite direction. After a 

 time the cone nuclei move to the external limiting membrane. 

 He used the silver-nitrate method. In my sections I am unable 

 to recognize at this age the two classes of bipolar cells. 



Chievitz ('87) says that, with the exception of the nerve-fiber 

 layer, the retina of the eighth-week human embryo is wholly 

 epithelial in character. At this age two layers of nuclei are seen. 

 An intermediate granular layer appears in the macular region 

 in the five months' fetus. This latter age corresponds well with 

 the development of the five-day sparrow. 



In the seven-day sparrow^ embryo the pigment of the pig- 

 ment epithelium is conspicuous and more dense than in the last 

 age. The granules are situated at the inner ends of the cells 

 at the lens region, but in the cells of the posterior part of the 

 retina they are diffuse and no longer located at the sides or outer 

 portions. 



The first appearance of the pecten is seen at this age. In 

 cross-section it is a conical projection of undifferentiated meso- 

 dermal cells from the optic disc. A detailed description of the 

 development of this structure is given in the discussion of the 

 pecten. 



The retina shows further advance in differentiation. The 

 various layers already described have become more noticeable 

 and extend almost over the whole of the retina. They, however, 

 still lack completion, being very indistinct. Between the outer 

 row of cells (the outer nuclear layer) and the pigment layer 

 in the axial region there now exists a narrow band-like region 

 free from nuclei. These are the dendritic processes of these 

 cells and represent an early stage in the formation of the rods 

 and cones. In the human fetus, according to Chievitz, the cones 

 begin to appear in the mq,cular region at the seventeenth vreek. 



