Biology Reference
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cell (the germinal cell) and how they are organised into ordered cell
tissues. The deterministic conception which predominates at the
present time is influenced by genetics and by the early work on
experimental embryology.
6.2.1
Embryonic induction
Driesch's experiments demonstrate that the differentiation of cells
cannot be solely the expression of their internal determinants (see
chapter 5, ยง5.3.1). An additional mechanism restricting their devel-
opment potential is therefore necessary and even if we reject
Driesch's and Elsasser's vitalism, we must explain it. Light has been
shed on this question by the work of Hans Spemann (1869-1941),
who performed tissue grafts inside embryos which demonstrated the
primordial role of interactions between cells. In the course of the
embryo's development, the cells have a mutual effect on each other.
What each cell becomes depends on the influences it receives from
the other cells, a phenomenon which has been called 'embryonic
induction' (Spemann, 1938; Bouwmeester, 2001).
Although Spemann's experiments did not in themselves indicate
the nature of the induction mechanism, it was immediately con-
ceived as a deterministic phenomenon. It was supposed that embry-
onic cells produce induction molecules which act on their neighbours
determining how they will differentiate (Saha, 1991).
6.2.2
The instructive model
The deterministic conception of embryonic induction gave rise to
what became called the 'instructive' model of cell differentiation. In
this model, cells differentiate because they receive 'instructions'
corresponding to signals (or information) carried by proteins. These
signals trigger cascades of reactions inside the cells (see Fig. 8) which
end in genes being activated producing cell differentiation.
In Fig. 19 cell B differentiates into cell D because it receives a
protein signal d synthesised by cell A. In the same way, cell A dif-
ferentiates into cell C because it receives a signal c synthesised by B.
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