18 WILLIAM H. F. ADDISON AND HAROLD W. HOW 



the lids form and separate is entirely apart from the visual 

 organ, yet the processes of development in the lids and in the 

 retina are so coordinated that their definitive end-results are 

 contemporaneous. 



DISCUSSION OF OBSERVATIONS 



In the albino rat two noticeable features in the formation of 

 the eyelids are the late period in gestation at which the eyelids 

 develop and the rapidity of their formation. The gestation 

 period averages twenty-two days, and not until the eighteenth 

 fetal day are the eyes covered by the lids (fig. 3). At the seven- 

 teenth fetal day the lids are represented only by ridges above 

 and below the eyeball (fig. 2), but so rapidly does the process of 

 formation proceed that in the space of a day the Hds have grown 

 across the eye, and have met and fused by their epithelial mar- 

 gins (fig. 4). As the hds approach one another in their growth, 

 the epithelium of the margin of the developing hds is the site of 

 especially active proliferation. In consequence of this, it grows 

 out noticeably beyond the mesenchymatous portions of the lids 

 (fig. 5), and first bridges the space between the approximating 

 hds as a thin epithelial membrane. As the mesenchymatous 

 portions of the hds continue to grow toward each other, this 

 thin epithelial membrane is pressed together and portions of it 

 are pressed aside as groups of epithehal ceUs (figs. 4 and 6), 

 which are soon lost. 



After fusion occurs, the junction, epithelium continues to 

 increase in the conjunctiva-skin dimension until about four days 

 after birth. There is a temporary diminution in this measure- 

 ment in the newborn, and this may be associated with the dry- 

 ing of the epidermis and the more rapid keratinization of the 

 superficial epidermal layers of cells then beginning. After the 

 fourth day the junction epithelium remains stationary until the 

 sixth or seventh day. During this time the measurement is 

 270-288m. Then, due to the deepening of the epidermal groove, 

 the length of this line of fusion becomes progressively less, and 

 at the twelfth day is only 75m . On the thirteenth and fourteenth 



