Morphogen gradient theories have enjoyed considerable popularity since the beginning of this century, but conclusive evidence for a role of morphogens in controlling multicellular development has been elusive. Recently, work on three secreted signalling proteins, Activin in Xenopus, and Wingless and Dpp in Drosophila, has stongly suggested that these proteins function as morphogens. In order to define a factor as a morphogen, it is necessary to show firstly, that it has a direct effect on target cells and secondly, that it (...) affects the development of target cells in a concentration‐dependent manner. With these criteria in mind, the evidence available for a variety of proposed morphogens is discussed. While the evidence is not conclusive in most of the cases considered, there is a strong case in favour of the three proteins mentioned above, which suggests that morphogens are potentially of general importance in controlling the development of multicellular organisms. (shrink)

Genetic and molecular studies of development in Caenorhabiditis elegans have identified regulators that appear to control pattern formation in the cellularized nematode embryo. Two genes, skn‐1 and pie‐1, are required for specifying the different identities of two sister blastomeres in a 4‐cell embryo, called P2 and EMS. The skn‐1 gene encodes a DNA binding protein that may control blastomere development by regulating transcription in EMS and its descendants. ABa and ABp, the other two sisters in a 4‐cell embryo, are influenced (...) to develop differently by cell signaling events that require the two genes apx‐1 and glp‐1. In this review, I summarize evidence that some or all of these genes may encode embryonic determinants of blastomere identity. (shrink)

The formation of segments in the Drosophila early embryo is understood in greater detail than any other complex developmental process. Now, by studying other types of insect embryo, we can hope to deduce something of the ancestral mechanism of segmentation and the ways in which it has been modified in evolution. The parasitic wasp, Copidosoma floridanum, is spectacularly atypical of insects in that the small egg cell divides extensively, with no initial syncytial phase, and forms eventually some 2000 embryos(1). This (...) process raises intriguing questions about the control of embryonic polarity and segmentation. (shrink)