July, 1923 ] 
ALLEN — POTENTIALITIES OF A CELL 
389 
tally, some fundamental features of cellular organization. The persistence 
of these fundamental features is consistent with marked alterations in 
the form, the internal structure, and the functions of the cell. The per¬ 
sistent, heritable features of organization are undoubtedly in large measure, 
in nucleated cells, especially characteristic of the nuclear substances, the 
cytoplasm being the medium through which from time to time varied 
sets of potentialities are expressed. However, the division of functions 
is probably not quite so sharp as this statement would imply; for some few, 
at least, of the hereditary potentialities of the cell seem to depend upon 
persistent features of the constitution either of certain parts of the cyto¬ 
plasm, such as plastids, or of the cytoplasm as a whole. 
The passage of the cell from pha.se to phase is conditioned by stimuli. 
This is true at least to the extent that, when a change is to take place, 
surrounding conditions determine just what that change shall be. Whether 
some change in the activities of the cell would occur if the environment 
remained unmodified, no one can say. Indeed, the activities of the cell 
necessarily modify its environment; so that an unchanging environment 
for a living cell is unthinkable. Experiment shows, however, that a very 
large proportion of the processes of change that constitute life are or can 
be brought about by environmental changes; and that, as between two 
or more possibilities at any point in the story, the environment largely 
determines which alternative shall prevail. To this extent, a particular 
potentiality may be described as the power of responding, by a certain 
activity, to a definite stimulus or group or class of stimuli. In recognition 
of this point of view, various writers have described all the characters of 
an organism as responses. 
But this statement, like most generalizations, may be too broad. That 
the organization of the living matter not only establishes certain potentiali¬ 
ties, but, in conjunction with the environment, plays a part in determining 
the sequence of expression of those potentialities, is suggested by various 
facts. One such fact, already cited, is that a cell, in any particular phase, 
can not pass indifferently into any other phase whose potentialities it 
possesses. Limitations of this nature result in the appearance of some¬ 
thing approaching a life cycle. For instance, from the multinucleate 
sclerotium of Myxochrisis may come, after cell division, thick-walled 
quiescent cells, flagellate cells, or amoeboid cells; a quiescent cell, forsaking 
its wall, may become flagellate or amoeboid; and a flagellate cell may pass 
into the amoeboid phase. In spite of a considerable range of alternatives, 
the story moves in a certain general course; for an amoeboid cell does not, 
as a rule at least, reverse the order and take on the flagellate or the quiescent 
form. It is too early to say what further variations of the history unusual 
conditions may bring about; but a general tendency toward something 
like a cycle seems manifest. As there is a plasticity in form and function, 
so there is a plasticity in the sequence of forms and functions; but both 
