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



constant at the inner angle of the Ud margin. By this compari- 

 son one may judge that the inner groove in the junction epithe- 

 Hum, which is so marked at late stages (figs. 12 and 13), is mostly 

 the result of the expansion of the lid itself. 



It is true that there is some formation of keratohyalin granules 

 in the superficial cells of the groove, but the changes do not 

 usually go on to complete cornification before the epidermal 

 keratinizing process reaches them, and so can help but little in 

 the separation of the lids. 



Previous investigations of the developing eyelids have been 

 made principally on human material, and of these the most 

 complete are the studies of Contino ('07) and of Ask ('08). 

 These contain many references to previous literature, beginning 

 with Bonders ('58) who was apparently the first to picture a 

 thin section across the fused lids of the human fetus. 



The observations on the eyelids of mammals, aside from man, 

 are comparatively few, and are for the most part incidental to 

 other studies on the developing eye. Ewetsky ('79) found in 

 fetuses of domestic cattle that the fusion of the lids took place 

 in fetuses of 5.5 to 6 cm. long, and that they separated in fetuses 

 of 40 to 47 cm. In his figures of the approximating lids he shows 

 the epithelium on the margins of the lids, growing out as a ridge 

 in advance of the mesenchymatous part of the lid, as is shown 

 in our figure 5. 



Seller ('90), who studied the development of the conjunctival 

 sac, also examined the eyelids of young puppies at one, four, 

 eight, and nine days after birth, as well as Talpa embryos. In 

 the puppies he observed the formation of a broad cornified cell 

 plate between the lids, leading to their separation. 



Nussbaum ('08), in the course of his resume of the process in 

 man, refers to having also examined mice of two stages — at two 

 days and at ten days after birth. In the two-day animal the 

 stratum corneum of the epidermis showed no depression over 

 the junction epithelium, but at the ten-day stage the process of 

 cornification had advanced into the lid fissure, forming a groove 

 which was funnel shaped in cross-section. 



