VIUUSLS, PROVIRUSllS AND THE CONFLICT OF SYSTEMS 



inasmuch as the specific properties of a tumour, fibrous, epithelial, 

 and so on, can be transmitted from one organism to another through 

 a constant lineage of cells by transplantation. 



In its physiological character of excessive growth and unlimited 

 mitosis, the animal tumour is analogous to certain conditions in 

 plants. But properties of increased and even excessive grow^th can 

 arise in plants by genetic changes of various kinds. In the liigher 

 plants development can be extended by extra mitosis in the pollen 

 grains, through the action of particular "polymitotic" genes or of 

 extra heterochromatic chromosomes. In Sorghum, as we have seen, 

 the nuclei formed by polymitosis kill the pollen grains by mere 

 exhaustion of materials. Here, then, a nuclear change leads to a 

 cancerous condition, doubtless by favouring excessive ribose nucleic 

 acid formation. In the yeast Torula utilis, on the other hand, treat- 

 ment by camphor produces a permanent genetic change, not affecting 

 the chromosomes, which, according to Thomas, increases the growth 

 rate and doubles the size of the cell. The yeast, then, must undergo 

 an induced plasmagene mutation. In a single-celled yeast or pollen 

 grain, high mitotic rate and high growth rate correspond with 

 tumour formation in many-celled animals, where changes from a 

 normal to the potentially unlimited type of growth can arise, and 

 be detected, only somatically. 



In this unlimited growth there is a continuous transition from 

 the encapsulated tumour or the wart, which is limited or merely 

 cut off from further growth by its owai effects, at one extreme, to 

 the other extreme of the malignant tumour whose cells are some- 

 times capable of metastasis or migration to new sites. Correspondingly 

 there is a transition in the type of cell division between the rapid 

 mitosis like that of embryonic tissue and the hurricane mitosis like 

 that found in the nucleic-acid-flooded nurse cells of plants and 

 animals, where mitosis is rarely completed in order, and a jumble of 

 haploid and polyploid cells is formed, strewn with fragments and 

 micronuclci. 



These graded differences probably do not depend on any genetic 

 distinction in principle. There is another type of difference, 

 however, which matters a great deal genetically: that is the mode 

 of transmission. There are three orders or levels of transmission 

 of tumours. 



212 



A 



