396 W. G. RTDEWOOD. 



Bradypus tridactylns. 



The hairs of Bradypus are oval in section, and exhibit a 

 central clear area and a darker marginal area (fig. 3). The 

 central area stains very faintly if at all with magenta, and 

 being brittle is apt to crack in the cutting. It is marked by 

 a small number of minute air spaces, the true shape of which 

 is fusiform. The long axis of each spindle is parallel to the 

 length of the hair, and consequently the transverse sections 

 of the spaces are larger or smaller according as they are cut 

 through the middle or near the ends of the spindles. The 

 outer substance stains deepl}'^, and is thickly marked with 

 dark granules, and exhibits at the same time two sets of 

 radiating lines — a set of very fine and closely set lines around 

 the outer edge, and a set of coarser and more irregular lines 

 branching out from the central mass. The average size of 

 the transverse section is 240^ x 145 ^t. 



The outer substance is a layer not represented, or at least 

 not in this form, in the hair of any other mammal. It does 

 not extend the full length of the hair, but stops short near 

 the free end, and is absent from the basal third of the hair. 

 In optical section (fig. 4, upper part) it exhibits an oblique 

 striation. The terminal portion of the hair (the " Endfaden " 

 of Welcker [17]) has the normal structure of a non-medullate 

 hair with a scaly cuticle, but at a certain distance from the 

 point the diameter increases quite suddenly by the addition 

 of this new layer (fig. 2). The diminution in the width of 

 the central core at this point is probably not real, but an 

 optical effect due to the refrangibility of the external layer. 

 The figure i*epresents an optical section, not an actual slice 

 taken from the middle of the hair. The basal third of the 

 hair is thin as compared with the distal part, and measures 

 only 64 fi across (fig. 6) ; it appears transparent when the hair 

 has been clarified and mounted whole. In addition to the 

 minute fusiform air spaces it fi-equentl^- has larger air-filled 

 cavities, blunt ended, and about 60 fi long and 6 /li broad. 

 The transverse section of this part of the hair is nearly 



