DEVELOPMENT OF EYELIDS OF ALBINO RAT 9 



Four-day rat 



Hairs are beginning to show on the outside of the Hd epider- 

 mis as well as on the other parts of the head and body, but these 

 are barely visible on casual inspection with the naked eye, and 

 in consequence the pink color of the skin is very slightly affected 

 by the white color of the sparse short hair. In sections, how- 

 ever, hairs are readily found projecting from the longest follicles 

 (fig. 9). At the margin of the lids it is noticeable that the 

 follicles are not as far advanced as in other parts of the Uds. 

 In figure 9, h, are shown hair follicles still in an early stage of 

 development from the junction epithelium. In some of the 

 follicles near the margins of the lids, as at the three-day period, 

 there is a beginning differentiation to form the shaft at the basal 

 end, and there is a keratinization process going on in the outer 

 ends of the follicles leading to their canalization. Where this 

 cornification is seen there the stratum granulosum of the epider- 

 mis dips inward and the stratum corneum of the epidermis 

 forms a narrow strand or plug where the outlet of the follicles 

 will appear. As we follow the further development of the 

 young hair folhcles, which are now attached to the junction 

 epithelium (fig. 9, h), it will be seen that when they reach this 

 stage of differentiation, just described for follicles on the epider- 

 mal surface, the expansion and separation of the lids will have 

 brought their points of attachment to the free margins of the 

 lids (figs. 12 and 13). So that when their hair shafts begin to 

 form and to grow out, they will find a ready passage to the 

 exterior. The junction epithelium is shghtly increased in its 

 longest diameter, now measuring 270 to 288//, while the short- 

 est distance between the two lids is practically the same as at 

 the three-day stage, viz., 40^. 



Five-day rat 



With the continued growth of the lid margins a slight change 

 in the form of the junction epithelium is now seen. In addition 

 to the deepening groove on the outer stirface there is now dis- 



