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



The hair-follicle buds are increasing in number and are seen 

 in various early stages of development. The oldest ones have a 

 papilla at their basal ends, but none yet show the hair shaft 

 within. Many of the vibrissae follicles, however, are com- 

 pletely differentiated and contain a well-defined hair shaft. 

 Examination of the outside of the head region shows that many 

 vibrissae are protruding above the surface of the skin and that 

 some of them are nearly 1 mm. in length. 



Measurements of the junction epithelium, both at this and 

 the preceding stage, show that the adjoining parts of the lids 

 have continued to grow and that there is an increase in the 

 dimensions of the junction epithelium as compared with the 

 nineteen-day stage. Measured from the basement membrane of 

 one lid to that of the other, it was 54/i, while in the conjunctiva- 

 skin direction it was 240At. 



Newborn rat 



In the newborn rat (fig. 8) the epidermis shows a differentia- 

 tion into stratum germinativum, stratum granulosum, and 

 stratum corneum. The stratum germinativum is composed of 

 a basal layer of cells with deeply staining nuclei and of several 

 rows of paler staining cells. The stratum granulosum is very 

 distinct, by reason of keratohyalin granules in the cytoplasm of 

 the cells (fig. 8, k), while the stratum corneum is composed of 

 flat scale-like cells. At the junction the epithelium consists of 

 cells similar, for the most part, to those in the stratum germina- 

 tivum, i.e., a basal layer of cells with dark staining nuclei on 

 each basement membrane and polyhedral vesicular cells inter- 

 vening. On the outer surface of the junction, however, the 

 stratum granulosum and the stratum corneum of the surface 

 epithelium continue uninterruptedly. 



The dimensions of the junction epithelium are found to be 

 less than those at the preceding stage. This is to be associated, 

 probably, with the change of environment at birth. The length 

 from the conjunctival surface to the outer limit of the stratum 

 granulosum measures 180/x, while at the narrowest point between 

 the two lids it measures 40/z. 



