Evolution of the mechanisms that establish the embryonic axes.

A long-standing debate in developmental biology concerns the extent to which embryos are largely 'mosaic' (cell fates are allocated by localization of maternal determinants that are inherited differentially) or 'regulative' (cell interactions determine cell fates). Generally, it has been thought that amniotes, especially birds and mammals, are at the extreme regulative end of the spectrum, whereas most invertebrates, lower chordates and anamnia are more mosaic. Various studies have identified additional differences, including egg size, the timing of zygotic transcription and the speed of development. However, new research is starting to reveal among the vertebrate classes an astonishing degree of conservation in the intercellular signalling mechanisms that regulate cell fate and embryonic polarity before gastrulation.