190 SPECIAL HISTOLOGY. 



membrane which I have discovered in the perfect hair (see § 60), 

 another, external laver of cells occurs on the hair-sacs, which 

 can generally be separated only in shreds with it, from the cutis, 

 rarely altogether : this I regard as the first indication of the 

 fibrous layers of the hair-sacs. In the 16th and 17th weeks 

 the processes of the mucous layer, which I will henceforward 

 simply call " hair-rudiments," increase in size up to "004 — 00C'" 

 length, and 0*03 — 004" breadth, and acquire thicker cover- 

 ings, but as yet exhibit no trace of a hair. In the 18th 

 week these first appear in the eyebrows, as hair-rudiments of 

 0*] — 0-2"' , their central cells becoming somewhat elongated, and 

 arranging themselves with their longitudinal axes parallel to 

 that of the rudiment, whilst the peripheral cells are disposed 

 with their now longer diameter transversely. A variety of 

 shade in the hitherto homogeneous hair-rudiment arises in this 

 manner, and a central substance, broad below, running above 

 into a sharp point, becomes marked off from an outer portion, 

 which is narrow below and thick above. When the rudiment 

 has attained a length of , 22'", this marking off is still more 

 distinct, the rather longer and especially broader, inner cone 

 having a somewhat clearer appearance (fig. 74). Finally, in 

 rudiments of hair of 0-28'", the inner cone is divided into two 

 structures, a central portion somewhat darker, and an external, 

 perfectly transparent and glassy, — the hair and the inner root- 

 sheath, — whilst the peripheral cells which have remained opaque, 

 constitute unmistakeably the outer root-sheath (fig. 75 A). 

 At the same time the papilla, which was even before (fig. 74) just 

 traceable, becomes more distinct, and the proper hair-sac also 

 more recognisable, as the cells which lie external to its struc- 

 tureless membrane begin to pass into fibres which may, even at 

 this time, be known by their decussation. The hair- sacs and 

 hairs arise, in other places exactly in the same manner as in the 

 eyebrows, except that their development takes place somewhat 

 later. In the 15th week, no rudiments of hairs are visible, except 

 on the forehead and eyebrows; in the 16th and 17th week they 

 appear all over the head, back, chest, and abdomen; and not till 

 the 20th week on the extremities. The hairs themselves never 

 make their appearance earlier than 3 — 5 weeks after that of the 

 rudiments ; in the 19th week, for example, the commencement 

 of hairs is nowhere to be seen, except on the forehead and 



