DEVELOPMENT OF EYELIDS OF ALBINO RAT 6 



This was found to be the condition in sections of fetuses from 

 three different litters, except in the case of two individuals. In 

 one of these (fig. 5) the development of the Uds was less advanced 

 and fusion had not everywhere taken place. The illustration is 

 taken from the middle region of the eyelids, and laterally to this 

 section the epithelial margins are in much closer approximation. 

 Evidently in the albino rat, as in other species, the middle region 

 of the lids is the last to completely fuse. As the figures show, 

 the two lids are of somewhat different proportions, the upper 

 being thicker and shorter and the lower more compressed and 

 longer. 



Microscopic examination of sections, from which figure 5 was 

 taken, shows that the skin epithelium now has four or five layers 

 of cells, the basal row being columnar in type, the remaining 

 ones flattened. On the margin of the eyelid the epithelium is 

 much thicker than elsewhere, and forms, in section, a conical- 

 shaped projection about IOOaj in length. The basal row of cells 

 is taller than on the neighboring skin and there are many more 

 rows of superficial , cells. These superficial cells are rounded or 

 polyhedral for the most part, arranged with their longer axes at 

 right angles to the margin of the lids, and are stained lightly. 

 On the external surface of the epithelial projection, however, the 

 cells are more flattened and are darkly staining, as in the super- 

 ficial cells of the epithelium of the skin. 



In another series of sections, from a fetus about eighteen days 

 of age, where the lids were in practically the same stage of devel- 

 opment as in this, the part played by the epithelial portion of 

 the eyelid in the primary closure is seen still more distinctly. In 

 the middle of the lid the epithelial margins are still separated 

 by a short space, but immediately adjacent to this region the 

 epithelium forms a thin narrow connecting membrane 350^t in 

 extent, between the mesenchymal portions of the two lids. 

 The plane of section cuts the lids shghtly obliquely, but in this 

 case, as in the preceding, the evidence is conclusive that the 

 epithelium grows out actively beyond the mesenchymatous por- 

 tions of the lid, to bridge the gap between the two approaching 

 lids. After the primary union is established, the framework 



