400 W. G. RIDEWOOD. 



seen to be empty tubes^ with a deeply staining lining. Tiiese 

 conditions do not appear to be pai*alleled in the hair of any 

 other animal. The branching tubes may possibly represent 

 a diffused medulla, for in most hairs the medulla stains deeply 

 and becoines largely infiltrated with air. This is the view 

 taken by Waldeyer (16, p. 187), who writes that the hair 

 shows " einen grossen centralen Mai-kstraug, der aber durch 

 Balken von Rindenschicht vielfach durchsetzt ist/' and by 

 Welcker (17, p. 55), according to whom " diese Markrohre 

 ist, wie bereits Erdl [3] erwahnt, innerhalb des dickeren Thiols 

 des Haars nicht circumscript, sondern in eigenthiimlicher 

 Weise mit Rindenschicht untermischt." Maurer's account of 

 the hair structure in Choloepus (8, p. 278) is as unintelligible 

 as his description of that of Bradypus. He speaks of the 

 cortex being thin in the broad part of the hair, thereby 

 implying that a compact central medulla is present. 



The cuticle is present, and it is imbricate, as can be seen 

 by the serrated appearance of the edge of the hair when 

 viewed in optical section. By staining rapidly, and washing 

 before the deeper parts of the hair have become affected, the 

 edges of the scales can be seen when the surface of the hair 

 is in focus. This is particularly the case with the hairs of 

 the under parts of the body, which have fewer longitudinal 

 grooves than those on the back. On the summit of the 

 ridges the cuticle is thick and highly refractive, but how the 

 cuticle is continued from one ridge to the next it is difficult 

 to determine. In very thin sections the cuticle can be traced 

 down the sides of the groove, becoming thinner and thinner, 

 and disappearing at the bottom. The grooves would thus 

 seem to be morphologically outside the hair. Yet it can be 

 seen in many places that the grooves are not perfect, as if 

 made with a plough, but are discontinuous ; and each portion 

 is canoe-shaped, open to the exterior at its middle, but covered 

 in at the two ends. Sections taken across the end of such a 

 segment of the groove show a continuous Cover of cuticle 

 (see a, fig. 8), and in surface view, with carefully stained 

 specimens, the edges of the cuticular scales can be traced 



