DEVELOPMENT OF EYE OF SPARROW 305 



lens, with the developing stroma of the iris. The pars caeca 

 can now be distinguished as pars ciliaris and pars iridica. Keibel 

 and Mall claim that the decrease in thickness of the pars caeca 

 is due to the more rapid growth of the ciliary body and the 

 ciliary processes. In other words, the retinal part does not grow 

 so fast, resulting in a drawing out of its surface. 



In the axial region the molecular layers are becoming more 

 distinct. The cells in the middle layer are thinning out by migra- 

 tion to either side. 



In the peripheral part of the retina little or no differentiation 

 can be discerned. The embryonic cells still occupy the whole 

 width of the retinal layer. 



The pigment layer shows a marked reduction in thickness 

 at the ora serrata and is now of almost uniform thickness through- 

 out (tables 4 and 5). 



In the axial region of the ten-day embryo the retina shows 

 much advancement in differentiation. The nerve-fiber layer is 

 wide and entirely free from nuclei. The ganglion-cell layer is 

 thicker and composed of two rows -of cells, which still retain 

 their embryonic form, but nerve-fiber processes could not be demon- 

 strated in the sections used. This layer is completely separated 

 from the inner nuclear layer by the inner molecular, which is 

 now free from nuclei in this region. Toward the periphery this 

 double row of ganglionic cells gradually becomes a single row, 

 then finally disappears by blending with the main nuclear mass, 

 due to the disappearance of the inner molecular layer. 



The outer nuclear layer is wider than in the nine-day chick, 

 and is composed of two rows of closely packed cells. This 

 layer is not so sharply marked off from the inner nuclear mass 

 as the ganglion-cell layer, because of a number of cells scattered 

 through the outer molecular layer. 



The outer limiting membrane is about 0.004 mm. outside the 

 outer nuclear layer. Beyond this is a light-colored layer 0.014 

 mm. wide which corresponds to the rod-and-cone layer. But 

 the rods and cones are as yet lacking in this region. All of 

 these layers become less and less distinct toward the periphery 

 of the retina, and finally blend with the embryonic nuclear mass. 



