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By Keith Stewart Thomson

At the present time developmental and evolutionary biologists are focussing renewed realization at the developmental process--those genetic and mobile elements that effect edition in person physique form or metabolism--in an try and larger know how evolutionary developments and styles inside of members will be constrained and regulated. during this vital paintings, the writer studies the classical literature on embryology, morphogenesis, and paleontology, and offers fresh genetic and molecular stories on improvement. the result's a distinct viewpoint on a suite of difficulties of primary value to developmental and evolutionary biologists.

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Jeffery and Meier, 1983; Cooke and Webber, 1983), there is no preferential distribution of mRNAs in the yellow crescent. There are, however, a dozen or so proteins, some connected with the cytoskeleton, that can be shown to be unique to the yellow crescent domain. In regulative development, layers of control processes are built up along with the layers of structural complexity because they not only produce this complexity but are produced by it. It is a spiral of complexity feeding into complexity, depending at all times on "whole-organism" properties as well as the separate regional specification of subparts.

However, control is possible through use of hybrids, where the paternal genome provides a marker that is only expressed when the zygotic rather than maternal gonome is involved. Once again the best data are available for sea urchins and amphibians. Gurdon's work on the ribosomal RNAs in Xenopus laevis shows that new populations of RNA come rapidly into existence at about the midblastula stage; see also Korn (1982). Galau et al. (1976), in a classic paper on the sea urchin, demonstrated that transcriptional activity begins very early after fertilization, but that translationa!

In the period immediately after fertilization, when the definitive cytoskeleton is still being assembled, there is a phase during which the organization of the egg can be continued under the guiding influence of the distribution of the yolk and other cytoplasmic factors by gravity. Gradually, however, the cytoskeleton takes over a fully controlling role, demonstrated by the fact that recovery from rotation and other experimental insults is not possible after the cytoskeleton is more complete (marked roughly by the time of gray crescent formation).

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