374 NORMAL HISTOLOGY. 



of the sac, and later is distinguished by the appearance of deeply- 

 pigmented granules, which mark the beginning of the pigment- 

 layer of the retina, to the formation of which the posterior lamina 

 of the optic vesicle is entirely devoted ; the pigment is first seen in 

 the vicinity of the lip of the cup, from which point the colored par- 

 ticles spread towards the posterior pole. 



The invaginated anterior lamina becomes greatly thickened and 

 differentiates into the remaining highly-specialized layers of the 

 retina. The process by which these ar^ formed corresponds in the 

 main points with the differentiation of the nervous centres, the re- 

 sulting tissues being of two kinds, the supporting neuroglia and 

 the nervous elements. 



The retinal lamina early presents a narrow inner zone, dis- 

 tinguished by its meagre nuclei as contrasted with the richly-nu- 

 cleated broad outer division ; this latter, next the pigmented 

 lamina, with many strata of nuclei, differentiates into an outer layer 

 characterized by small, deeply-staining nuclei, and an inner layer 

 of larger elements. The outer layer subsequently divides into 

 three strata, the outer nuclear, the outer reticular, and the inner 

 nuclear, while the inner layer produces two zones, the inner 

 reticular and the ganglion-cell. 



The rods and cones appear later as minute hemispherical eleva- 

 tions on the outer surface of the external limiting membrane, and at 

 first possess their inner segments alone, the outer members later 

 growing out from the inner. At birth in many animals (as cats, 

 rabbits, etc.) the rods and cones are wanting, and even in man they 

 are rudimentary ; the macula at birth is still undifferentiated. 



The nerve-fibres of the retina are derived probably from two 

 sources, from the neuroblasts of the retina itself and from those of 

 the interbrain. The hollow optic stalk becomes solid and con- 

 verted into the primary optic nerve, which acquires its nerve-fibres 

 from the ingrowing and outgrowing processes of the retinal and the 

 cerebral elements. 



The retinal blood-vessels develop within mesodermic tissue, 

 which spreads over the inner surface of the nervous layer at a com- 

 paratively late period ; the vessels first appear around the optic 

 nerve and spread peripherally. They are not connected primarily 

 with the central vessels of the retina, but with branches entering at 

 the periphery of the nerve (O. Schultze). 



The crystalline lens proceeds from the ectodermal vesicle 

 already noted. The walls of this sac very early exhibit marked va- 

 riation in thickness, the anterior lamina being relatively thin and 

 composed of a single layer of cuboidal cells, which persist as the 

 flattened polyhedral epithelium of the anterior lens-capsule. 



