recently wrote, 'we are not yet
within sight of being able to answer'. [15] But at least we can approach
it by a rough analogy. Let the chromosomes be represented by the keyboard
of a grand piano -- a very grand piano with a few thousand million keys.
Then each key will represent a gene or hereditary disposition. Every single
cell in the body carries a complete keyboard in its nucleus. But each
specialized cell is only permitted to sound one chord or play one tune,
according to its speciality -- the rest of the keyboard having been
sealed off by scotch-tape.*
* This sealing-off process also proceeds step-wise, as the hierarchic
tree branches out into more and more specialized tissues -- see
The Ghost in the Machine, Ch. IX, and below, Part Three.
But this analogy immediately poses a further problem: quis custodiet
ipsos custodes -- who or what agency decides which keys the cell should
activate at what stage and which should be sealed off? It is at this point
that the basic distinction between fixed codes and adaptable strategies
comes in once again.
The genetic code, defining the 'rules of the game' of ontogeny, is located
in the nucleus of each cell. The nucleus is bounded by a permeable
membrane, which separates it from the surrounding cell-body ,
consisting of a viscous fluid -- the cytoplasm -- and the varied tribes
of organelles. The cell-body is enclosed in another permeable membrane,
which is surrounded by body-fluids and by other cells, forming a tissue ;
this, in turn, is in contact with other tissues. In other words, the
genetic code in the cell -- nucleus operates within a hierarchy of
environments like a nest of Chinese boxes packed into each other.
Different types of cells (brain cells, kidney cells, etc.) differ from
each other in the structure and chemistry of their cell-bodies. These
differences are due to the complex interactions between the genetic
keyboard in the chromosomes, the cell-body itself, and its external
environment. The latter contains physico-chemical factors of such extreme
complexity that Waddington coined for it the expression 'epigenetic
landscape'. In this landscape the evolving cell moves like an explorer
in unknown territory. To quote another geneticist, James Bonner,
each embryonic cell must be able to 'test' its neighbour-cells 'for
strangeness or similarity, and in many other ways'. [16] The information thus gathered is then transferred -- 'fed back' --
via the cell-body to the chromosomes, and determines which chords on the
keyboard should be sounded, and which should be temporarily or permanently
sealed off; or, to put it differently, which rules of the game should
be applied to obtain the best results. Hence the significant title of
Waddington's important book on theoretical biology: The Strategy of
the Genes . [17]
Thus ultimately the cell's future depends on its position in the growing
embryo, which determines the strategy of the cell's genes. This has been
dramatically confirmed by experimental embryology: by tampering with the
spatial structure of the embryo in its early stages of development,
the destiny of a whole population of cells could be changed. If at this
early stage the future tail of a newt embryo was grafted into a position
where a leg should be, it grew not into a tail, but into a leg -- surely
an extreme example of a flexible strategy within the rules laid down
by the genetic code. At a later stage of differentiation the tissues
which form the rudiments of future adult organs -- the 'organ-buds' or
'morphogenetic fields' -- behave like autonomous self-regulating holons in
their own right. If at this stage half of the field's tissue is cut away,
the remainder will form, not half an organ, but a complete organ. If the
growing eye-cup is split into several parts, each fragment will form a
smaller, but normal eye.
There is a significant analogy between the behaviour of embryos at this
advanced stage and that very early,
Steve White
M. Lauryl Lewis
D. J. Molles
Brittney N.
Trevion Burns
Reba Taylor
Christa Lynn
Darien Cox
Heather Hildenbrand
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