DEVELOPMENT OF EYELIDS OF ALBINO RAT 21 



After the lids are fused they remain in this state until after 

 the arrival of the skin at the stage of cornification of its super- 

 ficial epithelial layers, the formation of the hair shafts, and 

 their appearance above the surface of the skin. In the rat this 

 is about seven days after birth and ends the stage of stationary 

 attachment. 



The final stage, that of gradual separation, is associated with 

 the maturing of the hair folHcles which have originated from 

 the junction epithelium. When the hair shafts of those which 

 are nearest to the tarsal glands have erupted at about 12 days, 

 the midHne of the persisting junction epithelium has become 

 cornified, and often in two days more final separation occurs. 

 The duration of this separation process in common with other 

 developmental processes is widely different in different species. 

 Thus, in the rat, where the animal attains a complete coat of 

 hair at about a week after birth, the separation process is com- 

 pleted at the expiration of another week or ten days. But in 

 man, where the lanugo appears during the fifth fetal month and 

 completely invests the body during the sixth month, disjunction 

 usually takes place at the end of the sixth (Contino, '07) or the 

 beginning of the seventh fetal month (Ask, '08). 



The period of attachment of the lids is related in time also 

 with the maturing of the retina. Functionally, the lids are a 

 protective part of the ocular apparatus, and their separation 

 does not take place until the retina has passed through its his- 

 togenesis. Thus, in the rat, at the time of formation and fusion 

 of the lids, the wall of the optic cup has a relatively simple 

 structure, with its cells arranged in two broad bands. During 

 the period of attachment of the lids differentiation proceeds, so 

 that at the time of disjunction, at about fourteen days, all the 

 layers of the mature retina are recognizable. The same holds 

 true for man, where at the time of fusion of the lids, about 

 seventy days (Contino, '07, and Ask, '08), the retina has a 

 simple structure corresponding to that of the rat at the eighteenth 

 fetal day (Bach and Seef elder, '14). Separation of the lids 

 occurs at the end of the sixth month or beginning of the seventh, 

 and by that time the retina has structurally all the elements and 



