302 JAMES ROLLIN SLONAKER 



The different layers of the retma are still too indefinite for 

 accurate measurement. Tables 4 and 5 show that the total 

 thickness of the retina in the axial region has increased to 0.138 

 mm., but at the lens it measures the same as the former age. 

 This is readily observed in figure 59, which represents a section 

 through the center of the eye of a seven-and-one-fourth-day 

 embryo. Some of the differentiation into layers above described 

 can be seen. The anterior margin of the retina is still in contact 

 with the lens and a differentiation into pars optica and pars 

 caeca has not yet occurred. 



At the eight day of incubation the thickness of the retina has 

 markedly increased in the axial region (fig. 64). This is mainly 

 due to an increase in the retinal portion. The different retinal 

 layers, with the exception of the inner and outer molecular 

 layers, have' now become sufficiently distinct to measure. These 

 measurements are given in table 4. The dimensions given for 

 the inner nuclear layer includes that of the two molecular layers. 

 At this age a space 0.016 mm. wide, and free from nuclei, is 

 situated between the single row of ganglion cells and the inner 

 margin of the retina. This is the nerve-fiber layer, but no nerve 

 fibers were visible in the sections used. 



A delicate line, consisting mainly of very small, brightly 

 stained bodies arranged in a row, runs parallel with the outer 

 margin and about midway of the outer clear zone (rod-and- 

 cone region). They appear to be situated on radial projections 

 passing through the outer nuclear layer. Some of these pro- 

 jections can be traced to cells in the middle nuclear mass. This 

 delicate line is the external limiting membrane and the radial 

 projections are the supporting fibers (Miiller's fibers). Accord- 

 ing to Ramon y Cajal, the supporting structures of the retina, 

 which correspond to the gha cells of the central nervous system, 

 develop at an early age. Their nuclei are at first scattered, 

 but later are grouped in the inner granular layer. 



Several blunt processes project from the cells of the outer 

 nuclear layer as far as the limiting membrane and other shorter 

 processes, of various lengths, are the forming rods and cones. 

 This corresponds to the ten-day chick embryo as described by 



