5OO TRANSACTIONS OF SECTION D. 
Developmental dynamics are set free, therefore, from the quite hopelessly com- 
plicated chemistry of the living substance. 
The processes of embryonic form-production are, as is well known, for the 
most part processes of evagination and invagination of epithelial cell-complexes— 
in other words, foldings of plates of cells. His has already made an attempt to 
refer these processes of folding to the effects of lateral pressures exerted by 
various rapidly growing neighbouring cell-territories. Roux has shown, however, 
that in the embryonic body the cell-layers which become folded also undergo 
their process of folding when they are cut out of their environment ; consequently 
the folding is not a passive process due to pressure of the surrounding cell- 
complexes, but must be localised actually in the cell-layers themselves that are 
being folded. Many other contradictions with dynamic anticipations have 
arisen from a consideration of the associations of the cells. 
It would, however, be quite erroneous to conclude from unfulfilled anticipa- 
tions of this kind that the application of physico-dynamic laws to the cell-layers 
is unsound; it is evident rather that the embryonic cell-layers cannot be inter- 
preted dynamically merely as flexible plates in the manner suggested by His. 
'Vhe cell-complexes are not sinyple dynamical systems, but have attained a high 
degree of complexity. The complication arises from the fact that each cell 
represents by itself a mechanical system complete in itself. To make use of a 
comparison, a plate of cells is not a simple wall built up of pliable material, as 
His believes, but an association of automobiles linked together one with another, 
each of which possesses its own driving force. Each of the automobiles can 
indeed react in a purely passive manner to external influences, if it does not 
puts its own motor in action; it can even heighten the effect of the external 
dynamical influence if it lets its motor work in harmony with the dynamical 
influence; it can, however, on the other hand, derange entirely the results of the 
dynamic influence by letting the motors of the various autos linked together 
run in different directions and with varying intensity. The dynamics of the 
cell-iayers involved in the embryonic processes of folding can therefore only be 
made intelligible on the basis of an accurate knowledge of cell-dynamics, the 
discovery of which is the object of the science of ‘cell-mechanics.’ Without an 
exact knowledge of the dynamics of the cleavage cells it is quite impossible to 
understand the dynamics of the cell-layers and of the process of form-production. 
Previous investigations of cell-mechanics, which owe their first foundations 
above all to Berthold and Biitschli, have shown that the dynamics of cells can be 
understood on the basis of purely physical laws, that fundamentally cells are 
even relatively simple dynamic systems, the results of which within the cell- 
complexes are only to be analysed with difficulty by the co-operation of many 
such dynamic systems, but are by no means inexplicable. The author now shows 
by some examples how the dynamic properties of cells, already established by 
the study of cell-mechanics, are able to elucidate physically in a simple manner 
whole series of phenomena exhibited by the behaviour of embryonic cell-com- 
plexes. In spite of such elucidations, however, the fact must not be lost sight 
of that the science of cell-mechanics can never extend the range of its explana- 
tions beyond the physics of the developmental processes alone; it is fully con- 
scious that it leaves entirely unexplained the specific physiological-chemical 
structure of the living contents, which is subject to the physical form-produc- 
tion. But since the science of developmental mechanics covers in its range all 
the factors, including the physical, of embryonic form-production, it is dependent 
upon the help of cell-mechanics in following out its ambitious aims. The study 
of cell-mechanics prepares the way for that of developmental mechanics. 
20. The Method by which the Individual Organism becomes adapted to 
New Environmental Stimuli by Use-acquirement, and on the Origin 
and Dependence of Use-acquirements on Variation and Selection 
of Intra-cellular Units. By C.J. Bonn, F.R.C.S. 
There is a tendency to regard the germinal variations on which racial evolu- 
tion depends, and the somatic modifications on which one part of individual 
development depends, as two essentially dissimilar processes. 
