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



one-day intervals. For stages up to one week old parafRn 

 embedding was satisfactory, but after the appearance of the 

 hairs double embedding in celloidin and paraffin gave better 

 results. In preparing for embedding the material to show the 

 later stages, especially from seven days onward, sometimes the 

 lens was removed, and in some stages only the eyelids were 

 embedded. For routine staining hematoxylin and eosin were 

 used. 



Seventeen-day fetus 



At this stage there are protruding ridges in the position where 

 the lids are about to develop, but one cannot say, as yet, that the 

 lids have been formed. As a consequence, the eye is uncovered 

 (fig. 1) and shows plainly on inspection. In sections across the 

 eye region the ridges are seen, as shown in figure 2. When 

 examined under the microscope, both the epithelial cells of the 

 skin of the head and the epithelial cells covering the ridges above 

 and below the eye are found to be two or three rows in thickness, 

 but the epithelium at the margins of the ridges is thicker than 

 the epithelium elsewhere. This is because the cells of the 

 basal row have here a greater height than elsewhere, and are of 

 a distinctly tall columnar shape, while the cells of the basal row 

 in regions near by are of a lower columnar type. The cells of 

 the superficial row or rows everywhere are seen to be flattened 

 in form. Of course, at this stage growth is active everywhere 

 and mitoses are abundant, but at the time under consideration 

 the epithelium at the margin of the ridges has the appearance of 

 being the site of relatively greater activity than in the skin 

 near by. The margin of the ridge constitutes, in fact, an apical 

 line of growth. 



Eighteen-day fetus 



Inspection of fetuses at this stage (fig. 3) shows that the 

 eyelids have developed rapidly in the preceding twenty-four 

 hours and now cover the eyeball completely. Examination of 

 sections through the eye region (fig. 4) usually shows that the 

 two lids have already fused together by their epithelial margins. 



