A third group of cysts contains material which represents essentially a production from the wall, with a difference of composition dependent upon the nature of the wall. Such cysts give rise to tumors through the retention of their contents, and they are called retention-cysts or retention-tumors. In the wall of the cysts is a gland-tissue, which may line the surface or lie beneath. The glandular structures may be cutaneous, mucous, or represent a part of the great glands of the body, as the liver and kidneys. The atheromatous cyst of the skin, the mucous cysts of the gastro-intestinal mucous membrane, and the ovula Nabothi of the uterus are examples of the retention of secretion within glands. The dropsical dilatations of the antrum, the vermiform appendage, the uterus, the biliary and renal canals furnish instances of tumors resulting from the retention of secretion on a large scale. In the subsequent history of these retention-cysts the secretion may be modified chemically and physically; the cells upon the walls may be transformed from columnar forms into flattened and scale-like varieties. In time, the original secretion frequently becomes a watery fluid, resembling the contents of the hygroma previously mentioned. This grouping of cysts in contradistinction to fleshy tumors omits the consideration of a series of cystic tumors of enormous size, the multilocular tumors of the ovary. This class represents a more complex form of cystic growth--one whose tendency is toward the reproduction of cysts, to which the term cystoma is applied. The cystoma is the result of an active new formation of epithelium and connective tissue, and is classified as a variety of the epithelial group of tumors. Morbid growths, as distinguished from cysts, are divided by Virchow into the simple and complex forms. The former consist of a single tissue, the histoid tumors; the latter of several tissues suggesting an organ, the organoid tumors; while still others, in which the number and grouping of tissues is so complex as to simulate systems of the body, even monstrosities, have received the term systematoid or teratoid tumors. Virchow claimed that the growth of most tumors took place from the connective tissues, and that most of the organoid tumors, especially cancer, arose from the formative action of the connective tissue in the part where {117} it first made its appearance. The structure of cancer suggested an organ, as it consisted of collections of cells resembling epithelium, within spaces or alveoli whose walls were formed of connective tissue. The epithelioid cells of the cancer, as well as the connective-tissue corpuscles, were considered to arise from pre-existing cells of connective tissue.