ANIMAL TUMOURS 



In both bacterium and virus the living is copying the dead pattern, 

 a pattern provided presumably, as in the chromosomes, by a nucleic 

 acid template. But in the bacterium a gap is being filled in the 

 structure, whereas in the virus a part is being taken down and 

 replaced under the pressure of an excess of myxoma. Also the agent 

 in the virus is presumably ribose and not desoxyribose nucleic acid. 

 Thus at this molecular level we can again produce a pseudo- 

 Lamarckian effect. We can control heredity from the outside and 

 control it this time constructively. 



The second aspect of virus genetics concerns the variation of the 

 host. Variegated Ahutilon striatum and the green A. indicum were both 

 grafted by Baur onto different parts of a plant of a third species, 

 A. arhoreum. The virus passed from striatum through the arboreum, 

 without seeming to affect it, and entered the indicum, turning it 

 variegated. Thus the virus multiplied in arboreum enough to travel 

 through it, but not enough to injure it. Indeed it behaved as though 

 it were an ordinary cell protein of arboreum. But if A. arboreum is 

 replaced by a species o( Lavatera the virus cannot get across. Thus 

 different species or varieties of host react in widely different ways 

 with a particular virus, and in the intermediate case they may be 

 "carriers" which show no effect. The difference between carrier and 

 susceptible has been shown to segregate as a simple mendelian 

 difference in the potato. The difference between carrier and non- 

 carrier in the insect vector of Maize-Streak virus is likewise geneti- 

 cally determined. And in the Potato Yellow Dwarf virus there are 

 strains fitted to two different insect vectors. There is thus a triple 

 genetic adaptation of host, virus and vector, a relationship we shall 

 examine later. 



Animal Tumours 



The connexions between viruses and other cytoplasmic deter- 

 minants are revealed by the different modes of origin or causation 

 of animal tumours. These tumours, we must recall, are all abnor- 

 malities of development inasmuch as they lead to some degree 

 of return of differentiated or mature cells to an undifferentiated or 

 embryonic condition with rapid mitoses, large nucleoli, and high 

 nucleic acid and protein production. They are also abnormalities 

 of heredity: they arise from genetic changes of particular cells, 



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