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



tinctly seen a slight concavity of the conjunctival surface of 

 the junction epithelium. The actual dimensions measured from 

 the conjunctival concavity to the epidermal groove are, how- 

 ever, practically the same as at the four-day stage, viz., 270 to 

 288m by 40m. The conjunctival groove which is now first becom- 

 ing definite is due to the expansion of the substance of the lid, 

 while the junction epithelium remains stationary. This differ- 

 ence in the rate of growth is entirely responsible for the con- 

 cavity on the conjunctival surface of the junction epithelium, 

 and is partly responsible for the groove on the epidermal surface. 

 In the latter case, the gradual keratinization of the epithelium 

 is the other factor. This keratinization proceeds in the same 

 manner as in ordinary surface epithelium by the preliminary 

 formation of keratohyalin granules and final cornification of the 

 cells, and has two loci. One is in the midline of the junction 

 epithelium, forming a groove, and the other is in the mouths of 

 the hair follicles attached to the junction epithelium when they 

 are beginning to form hair shafts. The progress of the kera- 

 tinization in these two loci combines to effect the major part of 

 the disjunction process. The first, in the midline of the junction 

 epithelium, is a continuous process, depending probably on the 

 distance the surface epithelium is from the source of nourish- 

 ment derived from the vessels of the tunica propria, and the 

 second, in the marginal hair follicles, is associated with their 

 arriving at a certain definite stage in differentiation. The 

 resultant effect — the splitting of the epithelium — appears to be 

 due to the same keratinization process manifesting itself jointly 

 in these two situations. The time relations of the process in 

 the two situations are closely interrelated, with the canalization 

 process in the hair follicles usually slightly in advance. As a 

 marginal hair follicle approaches the condition where it is about 

 to become canalized, its point of attachment to the junction 

 epithelium is found near the outer margin of the lid. In con- 

 sequence, the line of canalization of the epithelium is usually 

 through the outermost part of the junction epithelium and often 

 ending in the outer groove. The presence of these little canals 

 in the outer part of the junction epithelium seems to lead the 



