398 W. G, RIDEWOOD. 



by Welcker (17, Taf. ii, fig. 11) of the young hair in its follicle 

 at a time when the extra-cortex is forming would appear 

 to allow of no alternative proposition. Yet Welcker was 

 disposed to regard the Belegschicht as a new tissue inter- 

 calated between the cuticle and the cortical rod (17, p. 44) ; 

 and the effect obtained by macerating the hair in water, and 

 thus causing a thin cuticular layer to peel off (17, Taf . ii, fig. 

 14), lends support to his view. But this effect is very possi- 

 bly due to the excessive cuticularisation of the outer parts of 

 the external cells, and not to any morphological distiiiction 

 of layers. 



Leydig (6, p. 687) took the extra-cortex, or at least a part 

 of it, to be the cuticle, for he observed that, contrary to the 

 generalisation made by Reissner and Reichert, the hair cuticle 

 does contain pigment granules in one mammal, namely, 

 Brady pus. Waldeyer (16, p. 186) supported, in the main, 

 Welcker's contention, and regarded the " Rindenmantel " as 

 a layer peculiar to the sloths, and lying below the cutioula ; 

 and Leche (6, p. 934) is probably only adopting Welcker's 

 suggestion when he remarks of the " Umkleidungsschicht " 

 that " sie besteht aus einer zwischen Cuticula und Rinden- 

 substanz gelegenen pulposen, lufthaltigen Zellenschicht." 

 Maurer (8, p. 278), on the other hand, holds that the thicken- 

 ing of the distal part of the hair of Bradypus is mainly 

 effected by the cuticle (Oberhautchen). His account, how- 

 ever, is very confusing, since he speaks of a medulla extend- 

 ing two thirds of the length of the hair, and of the cortical 

 cells being pigmented ; and although he gives the title of 

 Welcker's classical paper in his bibliography, he fails to 

 contrast his own observations with those which this author 

 had already placed on record. 



The biological significance of the extra-cortical layer is full 

 of interest, and has been made known by the writings of 

 Welcker (17) and Sorby (15). The layer has a tendency to 

 crack in a transverse direction, and in the cracks there come 

 to lodge unicellular algte, to which Kiihn (17, p. 66) has 

 given the name Pleurococcus Bradypi. The moisture of 



