DEVELOPMENT OF EYELIDS OF ALBINO RAT 



23 



hair shafts are visible on the outside. As is well recognized, 

 hair folhcles are the seat of numerous mitoses, and they are 

 evidently now able to help supply the necessary cells for expan- 

 sion and growth, each in its own local area. At any rate, it is at 

 this time that the junction epithehum begins to increase in its 

 interpalpebral dimension, and it is apparently this increase in 

 the mass of epitheUal cells which removes the cells of the midline 

 of the junction epithelium farther from the source of nourish- 

 ment in the mesenchymatous portion of the lids, and so favors 

 the progress of the keratinizing process. 



The tarsal glands are also considered to take a part in the 

 opening of the Hds, but they are found to participate to a com- 

 paratively shght degree in the albino rat. These glands have 

 their beginning in the twenty-first fetal day, as epithelial pro- 

 liferations from the conjoined epithelium, near the inner angle 

 of the eyehd (fig. 7, t). At birth they measure 70m m length 

 and at seven days have increased to 540m, but are still sohd 

 throughout. At nine days the ducts show a distinct lumen, but 

 it is not until the twelfth day that some of the ducts are found 

 to have a direct opening to the outside, while most of the duct 

 mouths are still obstructed by plugs of cornified epithelium. 

 It is just before this last stage that the changes in the develop- 

 ing duct outlet may play a part in aiding the separation of the 

 lids. For at this time the terminal part of each duct where it is 

 continuous with the junction epithehum is still a sohd mass of 

 cells, although the greater extent of the duct is canahzed. In 

 the cells lining the duct immediately adjoining the unopened 

 part some keratinization is going on, and as this process extends 

 out to meet the similarly changing cells of the junction epithe- 

 lium, the cornification of the conjunctival side of the raphe is 

 accelerated. Schweiger-Seidel ('66), who was one of the first 

 to study the process of lid separation in man, emphasized the 

 formation of secretion in the sebaceous and tarsal glands as 

 assisting in the opening of the hds, but this does not appear so 

 important in the rat. 



It is seen by comparing figures 7, 11, and 13 that the points ot 

 attachment of the tarsal glands to the surface epithelium remain 



