Early Pattern Formation
The processes of development form a continuum that begins with gametogenesis and ends only with the death of the individual organism. It is therefore artificial to try to define separate phases and stages of these processes, just as it is artificial to try to separate the structural history of the embryo into a series of discrete forms through time with discrete and definable properties (let alone trying to match such artifacts to putative phylogenetic stages). But at the same time, the sequence of mechanisms of development is hierarchically organized. The major early event is the transfer of control over development itself from the purely maternal factors inherited within the egg and particularly in the egg cytoplasm, to the switching on of the zygotic genome and transfer to zygotic control of cell function, interaction and differentiation, morphogenesis and cytodifferentiation. This transfer does not occur at a single instant, nor is it easy to generalize about it even with a single group of organisms. Other landmarks are harder to find, especially ones that can be compared consistently over a range of different organisms. However, one can roughly divide the processes of development, for the purpose of organizing a discussion at least, into two main phases: early and late pattern formation phases. Early pattern formation can be defined as that part of the developmental sequence in which all the major mechanisms that control the shaping of the embryo, both its morphogenesis and cytodifferentiation, are set into place. In a vertebrate, early pattern formation would be everything from gametogenesis up to and including gastrulation, by which time all the essential elements of tissue interaction that will cause the morphogenesis of the embryo have not only been regionally defined and correctly positioned, but have started to function. Late pattern formation comprises the stages of morphogenesis and cytodifferentiation. As we will see, morphogenesis itself can be divided into two stages, early and late: roughly speaking, in the earlier part, morphogenetic pattern-controlling mechanisms are set in place, and in later stages their results are expressed.