D.—ZOOLOGY 87 
ture of the egg, so that under natural conditions the cytoplasm possesses 
the properties of a solid rather than those of a liquid. If this be the case, 
we are faced with the striking fact that the centrifuged egg develops 
normally, so that any structure which is destroyed by centrifugal force is 
very rapidly regenerated spontaneously when the force ceases to be 
applied—such powers of spontaneous regeneration are unknown in the 
physical world. The evidence is, however, against the view that the low 
viscosity of cytoplasm is more apparent than real, and the suggestion is 
entirely inadmissible in respect to the nucleus for, in this case, the fluid 
nature is revealed without the application of any force other than gravity. 
If we base our conception of the structure of protoplasm on the facts 
revealed by physical methods, we must imagine a system of very great 
chemical complexity and of very great potentiality for spontaneous self- 
differentiation within a fluid framework. Protoplasm cannot be regarded 
as a fluid crystal, for it possesses dynamic properties which are constantly 
expressing themselves in a variety of ways. Two general conclusions 
seem possible. We may assume that the molecules of protein and of 
other substances in the cell are so arranged in respect to each other that 
they constitute a highly active chemical system, and that the mechanism 
which maintains this molecular orientation is such that individual mole- 
cules or groups of molecules are able to move in the way necessary to give 
fluid properties to the whole system but not free to distribute themselves 
at random. If this be the case, the whole cell must be regarded as a 
fundamental unit, whose organisation is such that its structure cannot be 
destroyed by centrifugal force. So far such an organisation is not known - 
in dynamically comparable systems of an inanimate nature—we must 
regard it for the time being as an attribute peculiar to the living state, 
and as an attribute which is as fundamental as any of those employed for 
the description of inanimate matter. An alternative view is, however, 
possible. 
We may look on a mass of protoplasm as a very fine emulsion, the funda- 
mental units of which are extremely-small. If we assume that the pro- 
perties of the system as a whole are essentially those of each individual 
unit, then we have no great difficulty in seeing how mass disturbances fail 
to affect the properties of the whole system. The displacement of the 
particles by diffusion, or other causes, throughout the mass of the system 
will not influence the fundamental properties of the cell or nucleus if these 
properties are essentially those of the small individual units. The con- 
ception of the living cell as an aggregation of a very large number of funda- 
mental units is in keeping with the fact that small fragments of egg-cells 
retain some at least of the properties of the whole system. It is also in 
keeping with the very small dimensions (as in viruses) within which 
living phenomena have been observed. ‘There is some evidence to support 
the view that single differentiated cells also represent aggregates of very 
small living units. For example, a suspension of the spermatozoa of the 
sea-urchin Echinus in sea-water, after a period of maximal activity, enters 
a phase of declining mechanical and respiratory activity. If we consider 
a single spermatozoon during this period of senescence, we find that the 
intensity of its mechanical and respiratory activity declines in a way which 
is characteristic of a population of units which differ from each other in 
