| Popis: |
The central target of contemporary biology is still the problem of cell differentiation. In many animal species — spanning a wide range from echinoderms to mammalians — the multi-cell organism originates, ultimately, from a single-cell: the egg. The process, whereby an egg starts dividing to give rise to an embryo, i.e. ‘cleavage’, can be triggered, in different species, by a variety of different treatments: fertilization by paternal sperm, artificial activation by exposure to acids or to temperature shock or even, in some case, simply by the pricking of a needle. Thus, it is the ‘egg-system’, and not only the DNA contained in the egg, that has the ‘know-how’ necessary for the making of an embryo. The egg, to say it with Max Hamburgh “far from being an ‘undifferentiated’ cell, is perhaps the most highly specialized cell of any organism”. |
