TUMORS OF DOMESTIC ANIMALS 37 
TERATOID NEOPLASMS (TERATOMA) 
Teratomas comprise a group of neoplasms containing hetero- 
geneous tissue elements of one or several mature tissues or organs, and 
seem to be derived from all three layers of the embryo. They are 
always of congenital origin and are usually cystic in nature. Der- 
mal structures are the prominent feature comprising skin, muscle, 
cartilage, bone, teeth, hair, also nerve tissue, and even certain of the 
viscera. The dermal tissues invariably predominate ; hence the name 
dermoid is generally used, and as the process is always accompanied 
by cystic change, the name dermoid cyst is used. When the cystic 
change is lacking and reproduction of the viscera and other periph- 
eral structures resemble fetal parts the growth is termed con- 
genital malformation or monstrosity. Some pathologists include in 
this group the so-called mixed neoplasms of congenital origin which 
have been previously mentioned as combinations of neoplasms. 
Dermoid cysts occur in man and are not uncommon in domestic 
animals. 
DERMOID CYST 
Small dermoid c}^sts occur frequently in dogs, horses, and cattle, 
in the order named. Not infrequently they are found in sheep and 
hogs. It is interesting to note that the hair fibers in the dermoid 
cysts in sheep resemble wool, whereas the fibers in the dermoid cysts 
of hogs resemble bristles, and the dermoids in birds frequently con- 
tain feathers. In humans (children) small dermal nodes occurring 
on the face are described by the name mandibular tubercles, In 
many mammals, especially dogs, similar mandibular tubercles or 
cutaneous nodes have been recorded. 
The most common place for dermoid cysts is the ovary. Some 
writers describe them by the name cystic embryomas. Less fre- 
quently are dermoid cysts found in the testicle. They are also found 
in the thoracic cavity, in the abdominal cavity, starting behind the 
peritoneum, involving the kidney, in the mesentery, and in the omen- 
tum. Dermoid cysts are occasionally found in the mammary gland, 
in the parotid gland, about the eyeball, on the head at the junction 
of the cranial bones, on the face, and on the neck. 
_ Dermoid cysts in the ovary, like those occurring elsewhere, are 
lined with epithelium. They generally contain hair, teeth, or other 
dermal tissues, and are filled with fluid, which may be clear or 
cloudy. The cyst contents may also be gelatinous, mucous, fatty, 
or sebaceous in nature. Microscopic sections show the structure of 
skin, hair follicles with hair, sebaceous and sweat glands, develop- 
ing and adult teeth, pharyngeal mucous membrane, intestine, and 
thyroid gland. Wilms reports finding traces of nerve tissue in ova- 
rian cysts. 
Most dermoid cysts are benign and grow slowly, but some of them 
are malignant and grow as rapidly as any malignant neoplasm, give 
metastasis, and recur after removal. 
. Some cysts in the ovary are not congenital, and are described as 
simple cysts or cystadenomas. They are covered by a connective- 
tissue capsule and are lined by columnar epithelium. Such cysts are 
variable in size, may become very large, and contain a thin, watery 
