I have always wondered how an multicellular organism develops into such a complex, but highly reproducible being.. now, I’ve been reading ‘Development of Multicellular Organisms’from the book: Molecular Biology of the Cell by Alberts et al. and it all seems so simple! (the basic ideas at least :D) The nematode worm C. elegans, for instance: The hermaphrodite exists of 1031 somatic cells and ~1000 germline cells, the male of 959 somatic cells and ~2000 germline cells. The entry point of the sperm will define the future posterior pole of the worm. Asymmetric divisions organize the cellular molecules in such a way, that at the 16-cell stage, the molecules needed for the germline cells are all located in a single cell (and remember that there is no transcription during these early stages of nematode development). Cross talk between cells creates new cell types, for instance: two cells types have developed next to eachother. One excretes “A” and the other excretes “B” the cells in the middle will be exposed to “AB” and thus start excreting “C” cell directly next to “C” now get an “AC” or an “CB” signal, and will again produce a different cell types like extraembryonic tissue, dorsal epidermis, neurogenic ectoderm, mesoderm. The HOX complex is interesting too, homeotic selector genes. It is a complex of genes on some chromosomes, and the genes are expressed sequentially according to their order in the complex. There are about 10 genes in one complex, the upstream genes code for the anterior of the animal, and the downstream genes for the posterior, nice graded in between. And then the mechanosensory bristle of the fruitfly (Drosophila), it is a complex structure of a neuron, covered with a sheet cells, in a shaft cell, held in place by a socket cell, where the mechanosensory bristle is an extension of the shaft cell. It turns out that these four cell types all originate from a single sensory mother cell. The division of that cell is assymetric, thus giving one cell the advantage over the other, that one will become the neuron. The neuron destined cell will inhibit the other cell to become neuron by lateral inhibition. Asymmetric cell division continues and the other cell types are born. So lateral inhibition forces cells to act in opposite ways. So basically a simple repetitave process that has different effects under different conditions creates all this complexity.