ard, and is known as a _scirrhous cancer_--a form which
is most frequently met with in the breast. If the cells undergo
degeneration and absorption and the stroma contracts, the tumour becomes
still harder, and tends to shrink and to draw in the surrounding parts,
leading, in the breast, to retraction of the nipple and overlying skin,
and in the stomach and colon to narrowing of the lumen. When the cells
of the tumour undergo colloid degeneration, a _colloid cancer_ results;
if the degeneration is complete, as may occur in the breast, the
malignancy is thereby greatly diminished; if only partial, as is more
common in rectal cancer, the malignancy is not appreciably affected.
Melanin pigment is formed in relation to the cells and stroma of certain
epithelial tumours, giving rise to _melanotic cancer_, one of the most
malignant of all new growths. Cyst-like spaces may form in the tumour by
the accumulation of the secretion of the epithelial cells, or as a
result of their degeneration--_cystic carcinoma_. This is met with
chiefly in the breast and ovary, and the tumour resembles the cystic
adenoma, but it tends to infect its surroundings and gives rise to
secondary growths.

_Rodent cancer_ originates in the glands of the skin, and presents a
special tendency to break down and ulcerate on the surface (Figs. 102
and 103). It almost never infects the lymph glands.

DERMOIDS

A dermoid is a tumour containing skin or mucous membrane, occurring in a
situation where these tissues are not met under normal conditions.

The _skin dermoid_, or _derma-cyst_ as it has been called by Askanazy,
arises from a portion of epiblast, which has become sequestrated during
the process of coalescence of two cutaneous surfaces in development.
This form is therefore most frequently met with on the face and neck in
the situations which correspond to the various clefts and fissures of
the embryo. It occurs also on the trunk in situations where the lateral
halves of the body coalesce during development. Such a dermoid usually
takes the form of a globular cyst, the wall of which consists of skin,
and the contents of turbid fluid containing desquamated epithelium, fat
droplets, cholestrol crystals, and detached hairs. Delicate hairs may
also be found projecting from the epithelial lining of the cyst.

Faulty coalescence of the cutaneous covering of the back occurs most
frequently over the lower sacral vertebrae, giving rise to small
congenital r