STRUCTURE OF THE HAIRS OF MYLODON LISTAI. 397 



circular (fig. 5). In all parts of the hair of Brady pus the 

 cuticular scaling can be seen by suitable staining and accu- 

 rate focussing. 



That the central part of the hair of Bradypus is a cortex, 

 and not medulla, as Eble supposed (2, Bd. ii, p. 440, and 

 Taf. X, fig. Ill), and that the peripheral part is extra-cortical, 

 has been ably shown by Welcker (17), who applied the name 

 " Belegschicht " to it. The relation which the extra-cortical 

 layer bears to the normal cuticle is very diflScult to determine. 

 A careful examination of the part of the hair Avhere the 

 transition occurs between the normal terminal portion and the 

 part provided with the extra-cortex (fig. 2) shows that the 

 arrangement of the imbricate scales of the cuticle is con- 

 tinued without interruption upon the exterior of the extra- 

 cortical layer, thus seeming to show that the cuticle is con- 

 tinued over the outer surface of this layer. The extra-cortex, 

 however, is very friable in old hairs, and comes away readily, 

 leaving the central column of cortical substance bare; and it 

 is then seen that the surface of the column is mai'ked bylines 

 taking a more or less transverse course, and suggesting 

 forcibly that the cuticular scaling is continued on the surface 

 of the cortex beneath the extra- cortical layer. There is yet 

 a third possibility, which may eventually prove to be the 

 correct interpretation, since it accounts for both sets of 

 appearances. It is that the extra-cortical layer is the cuticle 

 itself, enormously thickened and distinctly cellular, instead 

 of more or less homogeneous and structureless. The 

 arrangement of the cells would account for the markings on 

 the external surface of the hair, and the scaly appearance of 

 the cortical rod when laid bare would be due to the impress 

 left by the extra-cortical cells. The appearances presented 

 by that basal part of the hair where the extra-cortex is just 

 dwindling away certainly favours the third supposition. The 

 cells of the extra-cortex get thinner and thinner, and come to 

 resemble the scales of the cuticle. They become more firmly 

 adherent to one another and to the cortex, they appear more 

 homogeneous, and they stain less deeply. The figure given 



