Later, partitions form, and the embryo becomes properly cellular. Through all this, as I say, the original chemical gradients persist. It follows that cell nuclei in different parts of the embryo will be bathed in different concentrations of key substances, corresponding to the original two-dimensional gradients, and this will cause different genes to be turned on in different cells (we are now, of course, talking about the embryo’s own genes, no longer the mother’s). This is how differentiation of cells begins, and projections of the principle lead to further differentiation at later stages
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