HAIR STRUCTURE OF THE MONOTREMATA AT7 
tween the scales of the proximal and the distal portions of hairs 
is due to increasing amounts of attrition and consequent removal 
of material from the free distal edges of the scales. On the shield 
itself their edges are also much worn and smooth (fig. 14). This 
condition is best illustrated in the shields of the hairs from the 
tail and feet (figs. 5 and 6). Seales from different regions of the 
shield hair are shown in figures 13 to 17. 
The distinguishing feature of the cuticle of the shield is, how- 
ever, its greater thickness on the ectal than on the ental surface, 
the difference being due, possibly, to a demand for maximum 
protection where maximum attrition occurs (fig. 19). 
The medulla of the shield hair. The medulla of the shaft, in its 
characteristic appearance, is shown in figure 13. Toward the 
base of the shaft (fig. 12, Q) it pinches out, existing only as iso- 
lated, elongated cells. This it also does at the isthmus, but re- 
appears again faintly in the base of the shield (fig. 14). About 
two-thirds of the way toward its tip it gathers together again in 
a well-defined band, but with a different structure from that 
which it possessed in the shaft (fig. 17). The region of this re- 
appearance of the medulla in the shield is not always the same in 
different hairs; sometimes it is found nearer the tip, sometimes 
nearer the middle. The relation of the medulla to the cortex in 
the shield and shaft is shown in figures 13 to 18. 
Upon a gross examination the medulla of the shield appears 
similar to that of the shaft, but a minuter study reveals the fact 
that the two differ markedly in their cellular elements. In the 
former the cells are globular, of various sizes, and are grouped into 
hotryoidal masses; in the latter they exist as discoid cells, of 
uniform size and shape, and are regularly superimposed (figs. 23 
and 24). 
The shield hairs of the feet and tail, which apparently consist 
almost wholly of the shield element, bear medullas which do not 
at all resemble those of the shjelds of the back just described. 
Although the cells are somewhat fragmentary, they resemble 
those of the shaft of the shield hair (figs. 26 and 27). Cross- 
sections of these hairs show that the cuticle is no thicker on the 
extal than on the ental surface. Here likewise they resemble the 
