DEVELOPMENT OF EYELIDS OF ALBINO RAT 11 



way for the deepening of the outer groove, and they serve as 

 numerous little independent centers from which the cornifica- 

 tion process extends into the adjoining junction epithelium. 



Six-day rat 



At this stage the epithelial proliferation at the margins of the 

 eyelids appears to have reached its maximum effect in the pro- 

 duction of the junction epithelium. This is shown by the dimen- 

 sions of the junction epitheHum, which now measures 288At X 

 40/i. From this time onward it will be seen that the actual 

 measurement of the junction epithelium diminishes in its con- 

 junctival-epidermal dimension, and, conversely, from the eighth 

 day onward, the interpalpebral dimension increases. Hence- 

 forth, although epithelial proliferation and growth continue at 

 the margins of the hds, no epithelium is added to the length of 

 the midline of the junction epithelium. Since, however, the 

 epidermal and follicular keratinization continues, the deepening 

 of the epidermal groove continues, and the result is the begin- 

 ning of the disjunction process. 



Seven-day rat 



The conjunctival-epidermal length of the junction epithelium 

 is now slightly diminished as compared with that found at six 

 days, while the interpalpebral length remains the same. At the 

 outer angle of the lids are long hair follicles, some containing 

 distinct hair shafts. The tarsal glands have grown greatly in 

 length and measure 540^, but are still soHd structures. But 

 between the long well-developed follicles at the outer angle and 

 the tarsal glands at the inner angle are again seen small follicles 

 in early stages of development growing from the junction epithe- 

 lium. This area of the lid is evidently still growing and dif- 

 ferentiating and is still at an earher stage of its developmental 

 history than either the epidermal or conjunctival part of the lid. 

 As the succeeding stages are studied, it will be seen that the 

 disjunction process is concurrent with the further differentiation 

 and growth of this part of the lid. 



