The simplest is as follows: I. Histoid.

III. Teratoid - Mixed Tumors

Dermoids. Teratoma. Cholesteatoma.

IV. Chorio-Epithelioma, Syncytioma Malignum

Deciduoma Malignum.

Combinations of tumors that have been derived from the same blastodermic layer frequently occur, as fibrosarcoma, fibromyoma, etc. One type, however, cannot be transformed into another.

The following classification of Adami's is recommended as being the most logical on account of its having been based upon a careful study of the histogenesis of the tissues.

Adami explains the classification as follows: "We thus find that the embryo comes to exhibit cell collections of two orders, which may be termed 'lining membranes' and (for lack of a more expressive word) 'pulps,' the 'lining membranes' being the persistent epiblastic, hypoblastic, mesothelial, and endothelial layers, the 'pulps' being the main mass of the neuroblast (of epiblastic origin), the notochord (of hypoblastic), and the mesenchyme (of mesoblastic). And now, following up the development of these different cell collections, we observe that the adult tissues derived from these two series exhibit well-marked differences, so that we can divide adult tissues into two great groups - -the lepidic (from Xe7U9, , a rind, skin, or membrane) and the hylic (srj, crude undifferentiated material).

"The characteristic of the lepidic tissues is that the specific cells which give them their main features are arranged either in layers or clusters in direct apposition; they are not separated by lymph-spaces or by blood-vessels; they possess, nevertheless, a supporting framework or stroma of hylic tissue in which run the nutrient vessels. Of the hylic tissues, the features are the opposite: separating the cells there is a matrix of intracellular substance, either homogeneous or fibrillated, while lymph-spaces and blood capillaries tend to separate and run between the individual cells.

"If in the lepidic tissues there is a stroma of hylic tissues, so here in the hylic there always enters lepidic tissue in the shape of the living endothelium of the blood- and lymph-vessels. In either case the elements of the other order occupy a subordinate position. While some pathologists, like O. Israel and Buxton, have already noticed this distinction, the histologists and em-bryologists have laid little stress upon it. The more we study tumors, the more we realize the importance of the distinction".

On this basis we obtain the following classification of normal tissues:

I. Lepidic, Or Lining Membrane Tissues

In which the blood-vessels do not penetrate the groups of specific cells and in which there is an absence of definite stroma between the individual cells, although such stroma, of mesenchymatous origin, may be present between the groups of cells.

1. Epiblastic: Epidermis

Epidermal appendages of the hair, nails, enamel of the teeth, etc. Epidermal glands. Epithelium of the mouth and salivary glands. Epithelium and glands of the nasal tract and associated spaces. Epidermal portion of the hypophysis cerebri. The lens of the eye. Epithelium of the membranous labyrinth of the ear, anus, and male urethra (except the prostatic portion).

2. Hypoblastic

Epithelium of the digestive tract and glands connected with it. Specific cells of the liver, pancreas, tonsils, thymus, and thyroid. Epithelium of the trachea, lungs, bladder, female urethra, and male urethra (prostatic portion).

3. Mesothelial

Lining cells of the pleurae, pericardium, peritoneum. Specific cells of the suprarenals, kidneys, testes, and ovaries (Graafian follicles).

Epithelium and glands of the Fallopian tubes, uterus, vagina, vasa deferentia, vesiculas seminales, etc.

4. Endothelial

Lining endothelium of the blood-vessels and lymphatics.

II. Hylic, Or Primitive Pulp Tissues

Organs and tissues in which the special characteristic is that the specific cells lie in, and are separated by, a definite stroma, homogeneous or fibrillar, in which there may or may not be blood and lymph-vessels.

1. Epiblastic

Nerve-cells; neuroglia.

2. Hypoblastic

Notochord.

3. Mesenchymatous

Fibrous connective tissues, cartilage, bone, reticulum of lymph-glands, bone-marrow, fat-cells, involuntary muscle tissue, spleen, blood-vessels, blood-corpuscles.

4. Mesothelial

Striated muscle, including cardiac muscle.

"Following this scheme of classification of the normal tissues, we may now divide the tumors arising from the specific constituent cells of the various tissues into two main genera - the lepidic tumors or lepidomata, originating from the above 'lining membrane' tissues, and the hylic tumors (hylomata), originating from tissues derived from the embryonic 'pulp.' We can further distinguish two broad groups of lepidic tumors - the primary, those whose cells are derived in direct descent from the original epiblast and hypoblast, and the secondary, or transitional, whose cells are derived in indirect descent from the same - i. e., have in the course of development passed through a mesoblastic or mesenchymatous stage before coming to form portions of a lining membrane".

In the classification that follows the author has not followed the exact wording of Dr. Adami, but has introduced such modifications as shall be consistent with his own text:

I. Lepidic, Or Rind Tumors. A. Lepidomata Of The First Order. 1. Of Epiblastic Origin

Tumors (epitheliomata) whose characteristic constituents are overgrowths of tissues derived directly from the epiblastic "lining membranes" or epiderm.

(A) Typical

Papilloma.

Adenoma of the sweat-glands. Adenoma of the sebaceous glands. Adenoma of the mammary glands, etc.

(B) Atypical

Squamous-cell carcinoma.

Carcinoma of glands of epiblastic origin.

2. Of Hypoblastic Origin

(A) Typical

Papilloma of the digestive and respiratory organs and bladder. Adenoma of the digestive and respiratory tracts, thyroid, pancreas, liver, bladder, etc.

(B) Atypical

Carcinoma developing in the same organs and regions.

B. Lepidomata Of The Second Order Or Transitional Lepidomata. 3. Of Mesothelial Origin

Tumors (mesotheliomata) whose characteristic constituents are cells derived in direct descent from the persistent mesothelium of the embryo.

(A) Typical

Adenoma of the kidney, testicle, ovary, urogenital ducts; uterus, prostate. Mesothelioma - adenoma of the serous membranes of the pleura, peritoneum, etc.

(B) Atypical

Cancer of the above-mentioned organs; squamous endothelioma, so-called, of serous surfaces, epithelioma of the vagina; adrenal mesotheliomata, hypernephroma.

4. Of Endothelial Origin

Tumors (endotheliomata) originating from the endothelium of the blood- and lymph-vessels:

Lymphangio-endothelioma.

Hemangio-endothelioma.

Perithelioma.

Cylindroma.

Psammoma.

Cholesteatoma.

II. Hylic, Or "Pulp" Tumors

1. Of Epiblastic Origin.

Tumors whose characteristic constituents are overgrowths of tissues derived from the embryonic pulp of epiblastic origin.

(A) Typical

Neuroma.

Glioma.

(B) Atypical

Gliosarcoma.

2. Of Hypoblastic Origin

Tumors derived similarly from embryonic pulp of hypoblastic origin. Chordoma.

3. Of Mesenchymal Origin. A. Mesenchymal Hylomata

Derived from tissues originating from the persistent mesoblastic pulp or mesenchyme. (a) Typical. - Fibroma. Lipoma. Chondroma. Osteoma. Myxoma. Leiomyoma. Angioma. Myaloma. Lymphoma. (b) Atypical. - Derived from mesenchymatous tissues. Sarcoma. - Fibrosarcoma.

Spindle-cell sarcoma.

Oat-cell-shaped sarcoma.

Chondrosarcoma.

Osteosarcoma.

Myxosarcoma.

Lymphosarcoma.

Chloroma.

Angiosarcoma.

Melanosarcoma (debatable).

B. Mesothelial Hylomata

Tumors which are overgrowths similarly of tissues derived from embryonal pulp of definitely mesothelial origin. Rhabdomyoma.