500 



HISTORY OF THE HUMAN BODY 



seems entirely lacking in many mammals (Insectivora and 

 some rodents). In the anthropoids it is very pronounced, and 

 in man it is designated by a yellow color (hence "macula 

 lutea"). Certain birds possess two such areas, medial and 

 lateral. 



To understand the addition of the accessory organs and the 

 formation of the eyeball it is necessary to examine more 

 thoroughly the early stages in the formation of the optic cup. 

 The study of a few actual sections will show that the invagina- 



a h 



FIG. 135. Diagrams of the retina. 



^ (a) Section including the fovea, showing the separate elements. [From 

 GEGENBAUR, after RAMON v CAJAL.] (b) More conventionalized representation of 

 retinal layers. [After GEGENBAUR.] 



fov fovea; /, membrana limitans interna; II, nerve fiber layer; III, nerve cell 

 layer; IV inner granular layer; V, inner nuclear layer; VI, outer granular layer; 

 VII, outer nuclear layer; VIII, membrana limitans externa; IX, rod and cone 

 layer; X, tapetum. 



tion of the primary outpushing is not a symmetrical one, but is 

 so effected that the cup is deficient for a little space on the ven- 

 tral aspect, and that this deficiency is continued as a groove 

 along the lower side of the optic stalk. It thus happens that 

 when the lens, which at this time is added to the optic cup, be- 

 comes closely applied to its rim, a fissure or oblong aperture, the 

 chorioid fissure, is left, through which communication may be 

 made with the interior of the cup behind the lens. Through 

 this inlet migrate embryonal connective tissue cells (mesen- 



