ADVANCES IN VIRUS RESEARCH—STANLEY Pail 
which have been purified possess varied shapes and form an almost 
continuous spectrum of sizes. The smaller rod or spherically shaped 
viruses appear to be simple nucleoproteins, some of which can be 
obtained in crystalline form. These appear to have chemical and 
physical properties which, neglecting virus activity, would tend to 
place them in the molecular world. The larger viruses have a com- 
position and properties which are characteristic, not of molecules, but 
of organisms. The viruses have certainly provided a link between the 
molecules of the chemist and the organisms of the biologist. Yet 
there is no place at which a line can be drawn dividing the molecules 
from the organisms. 
The viruses appear to form a continuous series with respect to 
structure, ranging from the smaller viruses, which are simple nucleo- 
proteins with many properties similar to those of ordinary molecules, 
on through viruses with a gradually increasing complexity of structure, 
to the larger viruses, which, with respect to structure and properties, 
are similar in many respects to organisms. However, it must be re- 
membered that the properties of only a relatively few purified viruses 
have been determined. In view of the possibility that these represent 
the more stable and more easily purified viruses, one cannot be certain 
that a true picture of the chemical and physical properties of viruses as 
a whole has been obtained as yet. Information regarding the mode of 
reproduction of viruses is needed most urgently. At present it is not 
known whether viruses reproduce by fission or by means of some new 
process. The solution of this puzzle would certainly represent a most 
important and significant advance, for the basic reactions character- 
istic of virus reproduction may well represent the fundamental process 
which characterizes all living things. 
