Advances in Cancer Research 7. [antikvár]

J.W. HEARD I. Introduction The virus-induced tumors of the chicken constitute a group of neoplastic diseases of various pathologic aspects ranging from complex leukcmias and associated conditions to simple fibrosarcomas. As a matter of general jiractice, it has been customary to consider the growths in two separate categories, in one, the diseases of the blood forming elements and, in the other, the connective tissue tumors usually designated as the sarcomas. Despite great apparent differences in pathogenesis and etiology of the growths, numerous findings have consistently suggested a distinct continuity of interassociations, both in the natural occurrence and in the viral transmission of the various neoplastic states. This has been emphasized and increased by more recent experimental demonstrations of unquestionable interrelationships among the etiologic agents themselves. For these reasons, it is expedient, as a basis for discussion and further inquiry, to regard the whole array of chicken tumors as a complex of neoplasms caused by a family of agents closely related in their general physical, chemical, and antigenic features. Comprehensive investigations of the chicken tumors and their causative agents were long impeded by the lack of tools and techniques adequate for a systematic approach to the problem. In retrospect, it is evident that the early demonstration of the viral etiology of the growths was untimely with respect to immediate exploitation of the potentialities for clarification of problems peculiar to virus neoplasms. Actually, these viruses were among the very first of the newly recognized filterable agents to be implicated in the etiology of diseases of previously unknown cause. It may be recalled that at the time, and for nearly thirty years afterward, instruments, particularly ultracentrifuges which are essential for direct investigation of the viral agents themselves, simply were nonexistent. Because of this, the studies were limited essentially to the biologic manifestations of pathogenesis, transmission, and immunology. These limitations raised many questions impossible of solution without definite knowledge of the nature of the agents. Such methods were relatively effective in the elucidation of some questions of the phenomena of other virus diseases, but they were insufficient to cope with many features of singular behavior of the neoplasms. A notable trend in the recent investigation of these avian tumors has been a shift in principle away from the earlier purely qualitative biologic experimentation toward the application of methods designed to afford quantitation and definitive correlation of data of diverse sorts. In tlie main, this has meant no more than an extension to the tumor viruses of