DEVELOPMENT OF EYE OF SPARROW 291 



The pigmentation appears as long, slender, branching strands 

 that have no connections with other strands. Some are short 

 and devoid of branches and others much extended and branched, 

 anastamosing with branches from other strands. This tends to 

 give the fenestrated arrangement of the pigment in the adult. 

 The spaces intervening are filled mainly with blood-vessels. 

 At this age the cornea, iris, and aqueous chamber have practically 

 the form of the adult. The ciUary muscles are partly developed, 

 some of the cells showing striations. 



The scleral plates can now be discerned with a fairly sharp 

 outline. They, however, still lack complete formation. The 

 cartilage layer is now composed of true cartilage, as in the adult. 

 It is thickest at and near the anterior margin. A much thicker 

 mass of cartilage forms a ring around the optic nerve entrance. 

 The thickest part of this ring measures 0.072 mm. At a distance 

 of 0.850 mm. from the nerve entrance the cartilage is reduced 

 to 0.020 mm. It thins out much more abruptly on the anterior 

 side of the nerve. The cells forming the fibrous portion of the 

 sclerotic coat are more densely arranged than in the earher ages. 



Four days after hatching (pi. 15 and figs. 92 and 93) the pig- 

 mentation of the chorioid has become more branched and dense, 

 resembling that of the adult. The scleral plates are well formed, 

 but do not extend backward to overlap the anterior portion 

 of the cartilage of the sclerotic coat as in the adult. The ciUary 

 bodies and muscles have about the normal adult appearance. 



From this time on to the adult condition little change is 

 noticed, other than completion of the development of the struc- 

 tures already noted. The chorioid becomes more pigmented 

 and vascular. The outer portion of the sclerotic becomes more 

 condensed and firm. The scleral plates change to true bony 

 tissue and extend backward to overlap the margin of the carti- 

 laginous portion of the sclerotic coat. Some changes in thick- 

 ness are also noticed. These are tabulated in table 3. 



