TUMORS. 343 



entiation of the parent-tissue. This failure in structural differen- 

 tiation may be so great that the resulting tumor resembles embryonic 

 tissue. Such tumors are clinically malignant, and, in general, it may 

 be said that the degree of malignancy is approximately proportional 

 to the lack of specialization exhibited by the formative activities of 

 the cells. Up to this point we have considered two possibilities in 

 the production of tumors : 1 . The production of a tumor by cells 

 which no longer respond to the needs of the organism in perform- 

 ing work for the general good, but which remain subject to the 

 influences controlling the structural differentiation of the parent- 

 tissue. 2. The formation of a tumor by cells which are less re- 

 strained by normal influences and which exercise their formative 

 powers without conforming to the special differentiation exhibited 

 in the parent-tissue. This we may regard as a reversion of the 

 cells to a less specialized state, in which they exercise their forma- 

 tive powers in elaborating tissues corresponding to those normally 

 present at some earlier stage in the development of the individual. 



There is a third possibility. The reversion just described may be 

 conceived as affecting the cells involved in tumor-production, but 

 those cells, instead of forming a tissue corresponding to the degree 

 of reversion they have suffered, may become specialized along some 

 divergent line of development and produce a tissue more or less 

 akin to that of the parent-tissue. Thus a tumor composed of bone 

 may be produced within some other form of connective tissue, such 

 as cartilage or fibrous tissue. The dissimilarity between the tis- 

 sues of a tumor and those of the part in which it grows would seem, 

 from this point of view, to depend upon the degree of reversion 

 that had taken place. Even after a tumor has once been formed, 

 portions of it may acquire a different structure, due to reversion on 

 the part of some of its cells or a modification of their formative 

 activities. There appears to be a limit to the extent of these rever- 

 sions. It is found in the early differentiation of the three embry- 

 onic layers. Cells derived from the mesoderm, for example, do not 

 seem to revert to such an undifferentiated condition that they can 

 develop tissues like those normally springing from the epiderm or 

 hypoderm. 



A still further complexity of structure may arise from the 

 formative tendencies of different cells within the same growth 

 developing along different lines of specialization. This occasions 

 the production of "mixed" tumors, composed of various tissues 



