Special systems, the classical approach

Abstract

In the preceding sections it has been assumed, more or less explicitly, that cell differentiation is fundamen tally a problem of differential gene activity i.e. different genes are active in different cell types but all cells have the same genetic information. In model systems such as the phage, sporulation and spore germination, where there is only one cell involved, the problem rnust clearly be one of time dependent changes in gene activity, but in multicellular, and especially embryonic systems, this is not so obvious. Indeed Weismann in the 19th century suggested that the origin of differences between embryonic cells had to do with differences in what he termed their ‘nucleoplasm’. The simplest hypothesis, he suggested, ‘would be to suppose that at each division of the nucleus, its specific substance divides into two halves of unequal quality, so that the cell-bodies would also be transformed ...’ Of course such an ordered parcelling out of genes amongst the cells of the embryo must leave the germ cells unaffected and so Weismann’s theory also necessitated a qualitative distinction between the somatic and the germ cells.