Biology Reference
In-Depth Information
Driesch performed an experiment similar to Roux's on a sea
urchin, but he obtained a completely different result. Instead of
inducing the formation of half an embryo, destroying one of the two
cells of the embryo ended in the formation of a complete animal,
though of a reduced size. Driesch repeated his experiment on
embryos at the four cell stage and was able to show that either a
single cell or a group of three cells taken together is capable of form-
ing a complete embryo. In the same way, half an older embryo con-
taining a thousand cells can produce a normal organism. These
results invalidated Weismann's theory. If cells resulting from several
successive divisions can form complete organisms, they must con-
tain all the germinative plasma, not just some of it. Driesch drew
an additional conclusion from these experiments, which demon-
strate that up to an advanced stage, one cell of an embryo can give
rise to all the cell lines of an organism, and that it possesses a
potential for differentiation greater than its actual vocation during
embryogenesis. By multiplying experiments on embryos taken at
different stages of development in a variety of experimental condi-
tions, he was able to verify this conclusion and demonstrate the
plasticity of cells, which enables them to adapt to these varied sit-
uations. The question then arose of how this immense potential for
development is reduced during embryogenesis so that only a single
potentiality is expressed.
To answer this, Driesch performed other experiments which led
him to formulate two further concepts. Firstly, the development
potential of all the cells from one region of the embryo seems to be
constant. The embryo is therefore 'an equipotential system'.
Secondly, to a certain degree, one region can be modified without
interfering with the development of other regions of the embryo.
The development of the various regions seems causally independent,
but nevertheless it ends in a harmonious organism forming an
organised whole. Driesch described this phenomenon as 'a harmo-
nious equipotential system'. In fact what he was describing corre-
sponds to essential properties which are nowadays completely
acknowledged, and which we now call cellular plasticity and robust-
ness of biological systems.
