768 The American Naturalist. [September, 
minute blood vessels, while here and there are developed mus- 
cle fibres of the smooth or involuntary type. This dermis is 
separated from the deeper tissues by a loose or aveolar con- 
nective tissue, in which fat is often developed to a considerable 
extent. 
In the development of a hair slight differences are observa- 
ble in different forms. In some the first phenomenon is the 
appearance of small papille in the dermis at the points at 
which hair is subsequently to be formed ; in others the pro- 
cess of hair formation is initiated by changes in the epidermis, 
which only appear after the formation of the papille in the 
first mentioned types. This change consists in an elongation 
of the basal cells in a direction at right angles to the surface 
where the hair is to appear (fig. 1), the 
result being that the epidermis becomes 
slightly pushed into the dermis in these 
areas. Beneath these thickenings, there 
next follows a multiplication of dermal 
ei ruS through Cells. As growth continues the inpush- 
es per stage of hair ings increase in extent, while the 
rmation in a mouse, after 
Maurer. At cisshownthe dermal cells arrange themselves around 
ET Pree . layer ” ingrowth to form a hair follicle with 
a slight projection, the hair papilla, at 
A base (fig. 2), the latter being supplied by a small capillary 
So far the epidermal ingrowth has been solid, but now a 
circular depression appears, which, 
deepening with time, separates a ẹ ; SIPS Tee 
central portion,therudimentary hair, 25 fa 
from a surrounding sheath (fig. 3). 
The relations will readily be seen 
from the illustrations, but a few 
- words may be added to make all clear. 
In figure 3 the hair is shown as a ne 
solid structure made up, as it pro- Kick r stages in t 
trudes from the skin, of three con- diversi of the hair in ihe 
centric layers; a central medulla, a prone enlier stage; on the 
middle so-called cortical layer, and lefta later stage: Ses! 
e, epi 
: ‘ hf, h P ilicle; Pp 
an outer cuticular layer. Inside the ae “a y, aie 
