1 I.—PHYSLOLOGY. 177 
given not only by filtration-data, but by other physico-chemical measure- 
ments, such as those obtained by Svedberg with the ultracentrifuge. It 
should be noted, as illustrating the difficulties of the problem and the 
uncertain meaning of some of the data, that Elford has regularly found a 
bacteriophage to be stopped by a membrane which allows the foot-and- 
mouth virus to pass; while, on the other hand, recent determinations of 
the rate of diffusion of bacteriophage, made by Bronfenbrenner, put the 
diameter of its particles at 0°6 of a millimicron, i.e. only about one-fifth 
of the accepted dimensions of the hemoglobin molecule. If we accepted 
such an estimate, we should be obliged to conclude, [ think, not merely 
_ that the bacteriophage is unorganised, but that its molecules are something 
much simpler than those of a high-molecular protem. It has even been 
suggested, though on very imperfect evidence, that it may be a moderately 
complex carbohydrate. Are we, then, to suppose that the foot-and-mouth 
virus is a similarly unorganised and relatively simple substance ? It is 
difficult, in view of the series of other agents, all conforming in many 
aspects of their behaviour to the classical type of the foot-and-mouth 
virus, and yet showing a range of dimensions up to that at which their 
units are apparently becoming clearly visible by modern microscopical 
methods. It will be clear, indeed, that, if we accept the lowest estimates 
_ for the size of the units of some viruses, such as the bacteriophage and the 
agents transmitting some plant diseases, we cannot by analogy apply the 
conception of their nature, thus presented, to viruses consisting of organisms 
_ which are ceasing to be even ultramicroscopic ; and we should be led to 
_ doubt the identity with the virus of the bodies which the microscope 
reveals. If, on the other hand, we regard the still invisible viruses, by 
analogy with those already seen, as consisting of even much smaller 
_ organisms, we can only do so by rejecting the conclusions drawn from 
some of the physical evidence. It is, of course possible that some of the 
_ agents called viruses are organisms and others relatively simple pathogenic 
_ principles in solution; but to assume at this stage such a fundamental 
_ difference, among members of a group having so many properties in 
- common, would be to shirk the difficulty. 
The third negative characteristic of a virus, viz., its failure to propagate 
_ itself, except inthe presence of living cells which it infects, may obviously 
again provide an unstable boundary, shifting with the advance of our 
_ knowledge and skill. We may regard it as not only possible, but even 
likely, that methods will be found for cultivating artificially, on lifeless 
media, some of those viruses, at least, which have the appearance of 
“minute organisms. Evidence in support of one claim to such success will 
probably be put before us. It would be playing with nomenclature to let 
nelusion in the virus group depend on continued failure in this direction. 
_ On the other hand, the dimensions assigned to the units of some viruses, 
_ tepresenting them as equal in size to mere fractions of a protein molecule, 
_ might well make one hesitate to credit them with the power of active self- 
_ multiplication. Experience provides no analogy for the growth of such 
a substance. by self-synthesis from the constituents of a lifeless medium ; 
_ the energetics of such a process might present an awkward problem. To 
account for the multiplication of such a substance at all, even in cells 
1931 N 
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