SPONTANEOUS NEOPLASMS IN MICE 171 



tlic animal a shrunken appt-arancc. With certain ncnjphisia there also 

 develop respiratory dilTiculties and in some cases marked edema. There is a 

 great variation in the speed at which different tumors grow, so that in some 

 cases the mouse may die from other causes without having been greatly 

 inconvenienced by a slow growing neoplasm. 



A vast amount of work has been done on the etiology but the exact cause 

 of cancer remains unknown (9, 11, 12, 37, 71). It is known that chronic 

 inflammation of either intrinsic or extrinsic origin may accompany the onset, 

 but chronic inflammation of itself is not enough to change normal body cells 

 into outlaw cells the chief function of which is unrestrained growth and 

 which exhibit a total disregard for normal tissue boundaries. Neoplasms 

 may develop from any cell, organ or tissue of the body which is capable of 

 growth. 



To summarize the characteristics of a neoplasm, we can say that it is an 

 autonomous new growth of undetermined origin which starts locally, serves 

 no useful function, may invade the adjacent tissues and even be transferred 

 to distant body regions of the host, grows progressively and, if uninterrupted, 

 eventually results in the death of the host. 



Tumors may be either fnalignant or benign. The benign forms are 

 homeotypic in structure, relatively slow growing, grow by expansion and are 

 encapsulated so that they do not infiltrate and do not metastasize. This 

 makes complete removal possible, in which case they will not recur. On the 

 other hand, the malignant forms are heterotypic in structure and possess no 

 capsules, so that infiltration and metastases are the most important charac- 

 teristics of this group. A benign tumor may develop into a mahgnant form 

 showing infiltration and metastasis. This is fairly frequently seen in 

 carcinoma of the breast in mice, where a small, partially encapsulated 

 adenoma may be continuous with an adenocarcinoma. 



Classification of Tumors 



There are several methods of classification of tumors (11, 12), but the 

 histological structure offers the simplest means, especially with small experi- 

 mental mammals such as the mouse. A tumor receives a name according to 

 the tissue which it most resembles. However, this applies best to only the 

 simple and benign tumors since many malignant forms do not resemble any 

 normal tissue. The terms sarcoma and carcinoma, therefore, have been 

 employed to designate the two main groups of the malignant neoplasms (11). 

 A sarcoma is a malignant tumor composed of cells of the connective tissue 

 type. It is formed on the connective tissue plan, developing its own stroma 



