TUMORS OP DOMESTIC AXIMALS 7 
mentary tissue, which forms every structure, organ, or system of the 
body. 
Virchow's classification is as follows : 
1. Histoid. Simple-tissue neoplasms. 
2. Organoid. Compound-tissue neoplasms consisting of several tissues like 
those found in organs. 
3. Teratoid. Mixed or congenital new formations, containing such structures 
as teeth, hair, cartilage, and bone. 
The following modified histogenetic classification is used by many 
authorities : 
1. Connective-tissue neoplasms. 
2. Epithelial neoplasms. 
3. Teratoid neoplasms. 
Adami (1) has suggested a modification of the histogenetic classi- 
fication based on embryonic layer formation, as follows : 
1. The lepidic group or lepidomas. comprising the lining membrane tissues 
or rind neoplasms, which are of epithelial and mesothelial structures deficient 
in stroma. 
2. The hylic or primitive pulp tissues of undifferentiated material, which 
are rich in stroma comprising essentially hypoblastic and mesoblastic structures. 
A careful study of the minute structure of neoplasms shows that 
all neoplasms are composed of elements present in the animal body 
in their adult or in their embryonal state. Neoplasms therefore 
originate from existing elements in the animal body and reproduce 
the adult or the embryonal tissue from which they have originated. 
Neoplasms, like normal tissues, consist of cells and a supporting inter- 
stitial ground substance. The cells of neoplasms, although retaining 
the morphological type of normal tissues sufficiently to be recogniz- 
able, are subject to considerable variation in size and shape. The 
greater their departure from adult cells the more pronounced be- 
comes the embryonal type of structure. The stroma is more abundant 
in neoplasms which originate from the mesoderm than in the 
neoplasms originating from the ectoderm or the entoderm, which 
have a very scanty stroma. 
"When a neoplasm is composed of a single tissue which is analog- 
ous to the tissue from which it started, it is known as a homologous 
growth, whereas those neoplasms which have departed from the 
normal tissue are described as heterologous. 
The histogenetic classification is followed in the arrangement of 
this bulletin and the various kinds of neoplasms in each class are 
discussed in order of their frequency of occurrence. 
CLASSIFICATION BY SHAPE (MORPHOLOGICAL CLASSIFICATION) 
Morphological classification is used to distinguish neoplasms by 
macroscopic conformation before the actual structure and nature are 
determined. It is as follows : 
1. Uniform swelling (affecting organs) : Goiter, lymphoma. 
2. Flat tabular swelling (sligbt elevations above the surface) : Keloid an- 
gioma, epithelioma, sarcoma of serous membranes. 
3. Xodes growing centrally which lie embedded in the tissue : Fibroma, 
myoma, adenoma. 
4. Xodes growing peripherally : Primary sarcoma, primary carcinoma. 
5. Tubers (partly protruding nodes) : Chondroma, osteoma, osteosarcoma. 
6. Fungus (spongy growth; : Telangiectatic sarcoma, cavernous angioma. 
