The original eukaryotic cell may have possessed the key processes necessary for metazoan development - cell differentiation, patterning and motility - and these are present in the cell cycle. Protozoa also possess key patterning processes. It remains a problem as to why there should be two main modes of development - one based on asymmetric cell division and the other on cellular interactions. The latter may be related to asexual reproduction.

The morphogenetic movements of gastrulation - as distinct from specifying the body plan - are highly conserved in a wide variety of organisms. This may reflect the requirement for patterning being specified in two dimensions, sheets of cells, and a third dimension being created by cell infolding.

The origin of the gastrula can be accounted for in terms of Haeckel's gastrea theory - an early metazoan resembling the gastrula. Gastrulation in Cnidaria may resemble the primitive condition but there is nevertheless considerable diversity. While this may reflect, for example, yolkiness, it seems that there is little selection on developmental processes other than for reliability.

Thus it is possible that the embryo is privileged with respect to selection and this may help account for the evolution of novel processes like the origin of the neural crest.

Reliability is the key demand made on development This may be provided by apparent redundancy. Since many developmental processes involve switches and spatial patterning reliability is provided by parallel buffering mechanisms and not by negative feedback.

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