DEVELOPMENT OF EYELIDS OF ALBINO RAT 5 



deep-staining, and shows abundant mitoses. The intermediate 

 layer is composed of one or two rows of pale, vesicular cells, 

 while on the surface are two or three rows of flattened cells show- 

 ing the beginning formation of keratohyalin granules. The 

 junction epithelium consists of the same three varieties of 

 cells, but arranged in the following manner. On the basement 

 membrane of each lid margin is a layer of columnar epithelium, 

 and, filling the intervening space between these two layers, are 

 rounded cells of a pale-staining, vesicular appearance. While 

 on the outer surface of the junction epithelium are two or three 

 layers of flattened, dark-staining cells, containing keratohyalin 

 granules, and continuous with the similar cells of the adjacent 

 skin surface. The progress of the keratinization process in the 

 superficial cells of the epithelial junction will constitute one of 

 the chief features to observe in watching the changes leading 

 up to the final disjunction of the lids. 



At the outer margin of the lids are found buds of hair follicles, 

 but as yet no distinct proliferation of epithelium to form the 

 tarsal glands has appeared. The muscle tissue of the lids is 

 now more apparent and the mesenchyme is becoming more 

 differentiated, so that it has a denser texture immediately 

 beneath the epithelium both of the skin and of the conjunctiva 

 with a looser texture intervening. Examination with the binoc- 

 ular microscope shows that a few of the vibrissae of the snout 

 region have already advanced through the epidermis and are now 

 apparent on the outside of the skin. 



Twenty-one day fetus 



At this age the beginning of the tarsal glands is shown by the 

 proliferations of epithelial cells at the inner angle of the margin 

 of the eyelid (fig. 7, t). Thus the tarsal glands follow closely, in 

 point of time, the appearance at the outer angle of the eyelid 

 of the hair-follicle buds which are noted at the twenty-day stage. 

 This was also the order of appearance observed by Ewetsky (79) 

 in fetuses of domestic cattle. He saw the anlage of the cilia at 

 the 6-cm. stage and of the tarsal glands at the 8.5-cm. stage. 



