540 THE HUMAN EMBRYO. 



anterior wall of the lens capsule. The cornea becomes trans- 

 parent during the fourth month, at which time it is strongly 

 convex, more so than in the adult. The cornea is very thick 

 from the first, and much thicker than the sclerotic; and at the 

 time of birth it is said to be absolutely thicker than in the 

 adult. 



The choroid is a very vascular layer, in which pigment 

 begins to appear towards the end of the second month. The 

 occasional occurrence in the adult human eye of a non-pig- 

 mented streak along the under surface of the eyeball, or even 

 of complete fissure of the iris, or coloboma iridis, is commonly 

 attributed to incomplete closure of the choroidal fissure. Inas- 

 much, however, as the choroidal fissure concerns primarily the 

 optic cup alone, and not the choroid coat, it is probable that 

 coloboma iridis should not be regarded as a simple case of 

 arrested development, but as involving some further pathological 

 process as well. The choroidal fissure normally closes about the 

 seventh week. 



The retina is formed, as in other Vertebrates, from the inner 

 and thicker layer of the optic cup. After its first establishment, 

 it grows for a time more rapidly than the outer coats of the eye- 

 ball, and consequently becomes thrown into folds which during 

 the second month project freely into the cavity of the eye. In 

 the later stages it becomes flattened out again. The rods and 

 cones are formed as outgrowths from the outer surface of the 

 thickened inner layer of the optic cup : they do not appear 

 until very late ; shortly before the time of birth. 



The mode of origin of the fibres of the optic nerve of the 

 human eye has not been determined with exactness. It appears 

 certain, however, that the nerve fibres are not formed out of 

 the walls of the optic stalk, and it is probable that they arise 

 as outgrowths from neuroblasts in the retina itself, which grow 

 inwards towards the brain along the path afforded by the optic- 

 stalk ; and on reaching the base of the thalamencephalon pass 

 across to the opposite side of the brain, thus forming the optic 

 chiasma, and continue their course up the sides of the brain as 

 the optic tracts, until they finally reach the corpora quadri- 

 gemina. 



The eyelids appear, towards the close of the second month, 

 as folds of skin above and below the eyeball (Fig. .213); they 



