196 EDWARD B. POULTON. 
PLATE 15. 
The figures in this plate illustrate the structure of the hairs and the struc- 
tures associated with them. Figs. 17, 18, and 25 represent structures in the 
mature animal; the remaining figures represent those of the young animal 
83 centimetres long, in which the large hairs alone had appeared above 
the surface of the skin. 
Fie. 17.— x nearly 200 diameters. Transverse section of a group of hairs 
from the middle of the back. The section is taken just below the epidermis. 
Groups like this apparently make up the fur which clothes the body of the 
animal, although modifications occur on the tail and limbs. The constituents 
of the group pierce the subepithelial tissue obliquely, and overlap each other 
on emergence. ‘Those in the lower part of the figure are overlapped by those 
in its upper part, which are situated anteriorly to them. Most anteriorly of 
all is usually found the duct (d.) of a gland resembling the Mammalian sweat- 
gland. It and the more deeply-placed gland-tube are found along the anterior 
margin of the outer root-sheath (0. 7. s.) of the large hair, or between it and 
one of the two anterior bundles of small hairs (Fig. 18, g.). Behind the duct 
is the most important member of the group—the large hair contained in its 
outer root-sheath (0. 7. s.). These hairs succeed each other very rapidly and 
always in an antero-posterior direction, so that the great majority of sections 
show two of them associated with one outer root-sheath. The shield-like end 
of the developing large hair (. 4.) is seen anteriorly in the lumen of the follicle, 
while behind it is the attenuated unpigmented basal part of a fully formed 
hair (/. 2’.) Behind, on either side, are two bundles of small hairs, each 
bundle being enclosed in a single epithelial ring. The majority of sections in 
each bundle are those of the attenuated pigmentless bases of fully formed 
hairs, while the minority are those of various parts of the shaft of hairs which 
are still growing from a bulb, and in these pigment is seen to be present. 
Fie, 18.— x nearly 200 diameters. A transverse section through a similar 
group from the same locality taken at a somewhat lower level. The duct has 
now passed into a gland-tube (g.) in which peripheral longitudinal smooth 
muscle-cells and central gland-cells are seen. The expanded end of the young 
large hair (¢. 4.) is here cut nearer to the bulb, and is only pigmented in its 
lower part. The medulla is distinct as a group of cells which still remain 
protoplasmic and stain deeply. After emerging at the surface these soft cells 
dry up, and give rise to the ordinary hair medulla with its enclosed air-bubbles. 
The cuticle 4o which the reference letters (/. 2.) point is seen to be far thicker 
on the exposed anterior surface than elsewhere. Comparison with Fig. 17 
shows that nearer the tip of the hair it possesses a uniform thickness, The 
base of the mature large hair (/’. #’.) is, at this deeper level, enclosed in the 
posterior wall of the outer root-sheath, and is irregular in outline. A little 
below this point it terminates in an irregular rounded extremity, sometimes 
