July, 1923] ALLEN — POTENTIALITIES OF A CELL 397 
rise to totipotent offspring. In this history there appears neither a funda¬ 
mental distinction between “ germ ” and “ somatic ” cells, nor a progressive 
distribution of potentialities during ontogeny to differentiating groups of 
tissue elements. These two conceptions, it may be said in digression, have 
found some seeming justification in animal embryology; partly by the 
unequal distribution of particular nuclear or cytoplasmic substances already 
referred to; and especially by the fact that tissue development in the more 
complex metazoa involves a progressive differentiation extending through 
a series of cell generations. The history in these organisms, under ordinary 
conditions, from obviously embryonic cell to fully differentiated tissue ele¬ 
ment includes the terms of existence of a succession of cells, being in this 
respect somewhat comparable with the less strictly defined life cycle of a 
many-phased flagellate; whereas in a plant the corresponding history is 
mainly or wholly included within the lifetime of a single cell. But despite 
this important difference, it is probable that the fundamental ontogenetic 
processes of the metazoa do not differ in principle from those here outlined 
for the vascular plants. The culture of isolated animal tissues promises 
to supply important evidence upon this question. 
The evolution of complex plants seems to have involved the loss of 
some of the potentialities that characterized their flagellate ancestors. 
The ability to revert to a flagellate phase, as shown in the formation of 
swarm-spores or motile gametes, has itself been lost—so far as we know— 
by the cells of conifers and angiosperms. It is noteworthy that in many 
ascending series, including those which led to the seed plants, this power 
was so long retained, even after incalculable ages of life upon land. The 
ability to pass into a naked amoeboid phase, still present in some of the 
Chlorophyceae, has also—again so far as we know—been lost by the cells 
of the more complex green plants. But if a few potentialities may be shown 
to have been lost, more have been gained. The widely varied types of 
cells found in the usually developed tissues of an oak, not to mention those 
of pathological growths, represent a vast number of potentialities which 
every newly formed cell of the tree possesses. Few of these potentialities, 
apart from those concerned in the more fundamental activities, were pos¬ 
sessed by the oak’s flagellate ancestors. The overwhelming majority have 
been added to the heritage of the cell. The acquisition of numerous new 
potentialities has characterized the evolution of the cells of complex organ¬ 
isms, both plant and animal. 
The responses which consist in the seriated expression of potentialities 
characteristic of the cells of any tissue are adjusted with remarkable deli¬ 
cacy to what must be minute differences between the stimuli that incite 
them. This delicacy of adjustment to varying stimuli is apparent if one 
considers two adjoining cells, starting with the same inheritance; one 
becoming, perhaps, an element of the cortical parenchyma, another a 
bast fiber or a sieve-tube element. It follows that, besides many new po- 
