170 GILBERT ©. BOURNE. 
cell-formation accompanies differentiation with growth of the 
mass, and why specialisation is not possible in continuous 
tracts of protoplasm. For, as Mr. Sedgwick himself admits, 
in a continuous mass of protoplasm, changes of molecular 
constitution in any one part would in time spread through 
the whole, so that a differentiation of one part would in time 
be impressed on all the other parts, and physiological division 
of labour would be out of the question. The fact that in the 
Protozoa there is differentiation within the limits of a single 
corpuscle presents no greater difficulty than the fact that in 
the epithelio-muscular cells of Ccelenterates, or the similar 
cells in Nematodes, there is differentiation within the limits of 
the cell. 
Again, metabolism in a large mass is greatly facilitated by 
its being broken up. As von Sachs says, “ It is very intelligible 
that not only the solidity but also the shutting off of various 
products of metabolism, the conduction of the sap from place 
to place, and so forth, must attain greater perfection if the 
whole substance of a plant is divided up by numerous transverse 
and longitudinal partitions into cell chambers.” The same 
thing applies, mutatis mutandis, to animals, and it is not 
difficult to see that the difference between holozoic and holo- 
phytic nutrition makes it impossible for the animal to grow 
to a large mass without division into cells, whilst such growth 
is possible in the case of plants which, like Codium and 
Caulerpa, live in water, or like Botrydium in damp earth. 
It is known that the spaces between epithelial cells which 
are traversed by the connecting strands of protoplasm, and 
were formerly supposed to be occupied by a cement substance, 
“are in reality lymph spaces, and this gives us some insight into 
the importance of the cell structure in animal organisation. 
The formation of cells with spaces between admits of nutrient 
fluid being brought to the very threshold of each constituent 
corpuscle of the organism. (See on this subject Th. Cohn, 
R. Heidenhain, Paulicki, Nicolas, Werner, and others.) 
Whilst the necessities of cohesion, solidity, and transmission 
of stimuli may explain the conjunct nature of so many tissue 
