MAMMALIA, 485 
order to increase the secretory surface. Their connection 
with the epidermis is, however, retained by the ducts which 
pass outwards, their cavities opening freely to the exterior 
on the surface of the skin. The great development of skin- 
glands is a marked feature of the Mammatia. 
We may distinguish two different kinds—(1) the sudorific 
or sweat-glands and (2) the sebaceous glands. 
1. The sudorific glands are developed by local ingrowth 
of the basal epithelium of the mucous layer. They lie 
deep in the dermis and excrete water, with inorganic salts 
in solution (sweat), discharged freely on to the surface of 
the skin. ‘he sudorific glands are of the tubular type, 
coiled and unbranched. 
2. The sebaceous glands are also produced from the 
basal epithelium of the mucous layer, but are only de- 
veloped in connection with hair-pits or follicles. Sebaceous 
glands are usually of the acinous or branching type, and 
they secrete sebacin, a fatty substance, the primary function . 
of which is to lubricate the hair. They also differ from the 
sudorific glands in being xecrobiotic, ¢.e., the sebacin is pro- 
duced from dead cells. 
Hair.—A hair is a structure found only in the Mammalia 
and it can only very doubtfully be compared with feathers 
or epidermic scales. It is essentially epidermic and its first 
trace in development is a small process or hair-germ formed 
from the mucous layer. This protrudes inwards into the 
dermis and elongates rapidly. Its base then becomes 
pushed into a pit within which the dermis protrudes, and at 
the apex of this pit the basal epithelium gives rise by rapid 
growth to a central axis of cells. The basal pit becomes the 
dermal papilla and the medullary axis gives rise later to the 
medulla of the hair. Around the medulla, between it and 
the basal epithelium, a thin layer or cylinder of the mucous 
layer becomes cornified, produced above the end of the 
medulla up to the surface of the corneous layer. Later on 
this cylinder divides into two so that a cylindrical cavity is 
produced. This cavity becomes continuous with the 
.exterior and terminates above the papilla. It differentiates 
the whole follicle:into a hair in the centre and the 7oot- 
sheaths around it. The basal epithelium, next the dermis, 
