Not change
of nutriment. The only visible or easily conceivable change was in
position relative to other cells of the embryo.
Let us in imagination simplify Driesch's experiment, for the sake of
gaining a clearer view of its meaning. In a certain embryo at an
early stage are certain cells whose descendants should form the
lining of the intestine and be used in the adult for digestion. A
second set of cells should form muscle endowed mainly with
contractility. When these two sets of cells, or some of them,
exchange positions in the embryo, they exchange lines of
development. The first set now form muscle, the second digestive
tissue. The only change has been in their relative positions.
Driesch maintains, therefore, that the goal of development in any
embryonic cell is determined not by structure or nutriment but by
position. And this would seem to be true of the cells of the
earliest embryonic stages.
Certain other experiments point in the same direction. Cut a hydra
into equal halves and each half will form a complete animal. The
lower half forms a new top, with mouth and tentacles; the upper
half, a new base. Cut the other hydra a hair's-breadth farther up.
The same layer of cells which in the first animal formed the lower
exposed surface of the upper half now forms the upper exposed
surface of the lower half.
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