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



result, one finds the hair shafts first showing in these youngest 

 follicles several days later than in the follicles elsewhere. Now 

 in the albino rat the ordinary hair follicles begin on the lids 

 during the eighteenth, nineteenth, and twentieth days, and the 

 largest of these first show shafts within at birth. These begin 

 to protrude through the skin at two and three days, and by the 

 end of a week after birth there is a complete covering of short 

 hairs. In comparison with this, we find that hair folhcles not 

 yet showing hair shafts within are found in connection with the 

 conjoined epithelium as late as eight days after birth. At nine 

 days these, too, begin to show differentiation of their internal 

 cells, and by twelve days the hair shafts begin to appear above 

 the epidermal surface. At this period a narrow band of kera- 

 tinized epithelium has developed in the midline of the junction 

 epithelium, and the final disjunction of the lids soon after occurs 

 by the splitting of this keratinized epithelium. So the time of 

 eruption of the youngest hairs at the margins of the lids is closely 

 connected with the time of keratinization of the middle of the 

 last remaining junction epithelium, which is the prelude to 

 final disjunction. 



From this survey it is seen that the period of time from union 

 to disjunction of the lids may be correlated with a fairly definite 

 period of development of the general integument of the body. 

 Thus, the time of formation of the lids is just prior to the 

 beginning of the epithelial proliferations which develop into 

 hair follicles of the ordinary small hairs of the skin. One has to 

 make an exception of the special large types of hairs, such as 

 the vibrissae of rodents and the hairs of the eyebrows, upper 

 lip, and chin of man. For in both rats and rabbits the vibrissae 

 follicles make their appearance just before the time of formation 

 of the eyehds. The above general statement holds true for man 

 also, for while we find that both the lids and the hair folhcles of 

 the ordinary lanugo have their beginning during the third 

 month of fetal life, the former practically always antedate the 

 latter. But an exception has to be made of the follicles of the 

 larger types of hairs of the eyebrows, upper lip, and chin, which 

 begin toward the end of the second month. 



