THE GENETICS OF SPONTANEOUS TUMOR FORMATION 273 



leukemic cells are not changed blood cells but are "a special race of cells hav- 

 ing independent origin." He states that they arise from reticulum cells 

 by focal proliferation. Migration obscures all trace of the point of origin. 

 By careful observation the earliest stages of this proliferation have often 

 been detected. They are so numerous that very clearly all of the early sites 

 of proliferation do iwl become sources of origin of the disease. This appears 

 to be evidence against the proliferation of normal lymphoid cells or organiza- 

 tions as being a precursor to the occurrence of leukemia. 



Evidence of extra-chromosomal influence. — It is interesting to note that 

 in somewhat the same manner as that reported for epithelial mammary 

 tumors, MacDowell (6g, 70) has recorded reciprocal cross differences in the 

 incidence of spontaneous leukemia in mice derived from a cross between a 

 "high" and a "low" line (Table 22). 



There is no doubt, therefore, that some influence which is extra-chromo- 

 somal in nature is operative. Further studies of the genetic behavior of 

 spontaneous leukemia should be important. 



Comparison of the Four General Types of Neoplasms 



Table 23 may help to summarize some of the main points of resemblance 

 and difference in the four main groups of neoplasms considered. 



BIBLIOGRAPHY 



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3. Andervoxt, H. B. axd W. J. McEleney. 1938. Influence of non-breeding 

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