THe SL Uy, OF *CANCER 17 
obviously of importance to determine experimentally whether 
or not all tumours tend to augment their energy of growth to 
the maximum exhibited by this group, and at the same time to 
assume the same histological garb. At present many tumours 
appear to retain their own peculiarities, and although the 
energy of growth may increase, it is not yet possible to deter- 
mine whether or not the tumours of one organ ultimately 
approximate to one form when growth is prolonged sufficiently. 
I may point out here that the first and second groups of 
tumours behave on transplantation as embryonic and adult 
tissues do, by exhibiting limited powers of proliferation, with 
the difference between cancer and embryonic tissue, that the 
former does not differentiate into an adult form. We are 
therefore able to place a positive value on the negative efforts 
which have been made to produce tumours by propagating 
normal tissues, and we are able to establish all gradations from 
the limited growth of normal tissues under experimental 
conditions, up to the remarkable phenomenon exhibited by the 
unlimited growth of the cells of some cancers under the same 
conditions. Thus experiment has bridged over the gulf between 
normal and cancerous tissue so far as their powers of growth 
are concerned. The limited growth of normal tissues, when 
transplanted, is independent of any extraneous organism. There 
is no need to assume such a dependence when cancerous tissues 
exhibit corresponding limitations; and if not, why should it be 
necessary to assume this intervention when the powers of 
growth pass insensibly through all gradations to those of 
unlimited amount ? 
The possibility of an accelerated rate of proliferation may be 
entertained when a tumour has been continually transplanted 
during long periods, as in the case of Jensen’s tumour. If I 
understand Ehrlich rightly, this is what he means when he 
states that the increase in “ virulence” is due to the avidity of 
the receptors having attained a maximum in consequence of 
rapid passage from animal to animal. I shall show immediately 
that there are natural fluctuations in the amount of pro- 
liferation. In the case of rapidly growing tumours all the 
inoculations which are successful yield tumours quickly (within, 
say, ten days). Mice then negative remain so. The fact shows 
that rapidity of proliferation is closely bound up with an extreme 
susceptibility to nutritive requirements. Since growth presents 
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