<P> Within the field of developmental biology one goal is to understand how a particular cell (or embryo) develops into the final cell type (or organism), essentially how a cell's fate is determined . Within an embryo, 4 processes play out at the cellular and tissue level to essentially create the final organism . These processes are cell proliferation, cell specialization, cell interaction and cell movement . Each cell in the embryo receives and gives cues to its neighboring cells and retains a cell memory of its own cell proliferation history . Almost all animals undergo a similar sequence of events during embryogenesis and have, at least at this developmental stage, the three germ layers and undergo gastrulation . While embryogenesis has been studied for more than a century, it was only recently (the past 15 years or so) that scientists discovered that a basic set of the same proteins and mRNAs are involved in all of embryogenesis . This is one of the reasons that model systems such as the fly (Drosophila melanogaster), the mouse (Muridae), and the leech (Helobdella), can all be used to study embryogenesis and developmental biology relevant to other animals, including humans . The fate map of the nematode (Caenorhabditis) can be analyzed down to the cellular level . This is due no cell mixing during development . What continues to be discovered and investigated is how the basic set of proteins (and mRNAs) are expressed differentially between cells types, temporally and spatially; and whether this is responsible for the vast diversity of organisms produced . This leads to one of the key questions of developmental biology of how is cell fate determined . </P> <P> The development of new molecular tools including GFP, and major advances in imaging technology including fluorescence microscopy, have made possible the mapping of the cell lineage of Caenorhabditis elegans including its embryo . This technique of fate mapping is used to study cells as they differentiate into their final cell fates . Merely observing a cell as it becomes differentiated during embryogenesis provides no indication of the mechanisms that drive the specification . Therefore, the addition of molecular manipulation techniques, including gene and protein knock downs, knock outs and overexpression, along with live cell imaging techniques has been transformational in understanding what mechanisms are involved with cell fate determination . Transplantation experiments are commonly used in conjunction with the genetic manipulation and lineage tracing . Transplantation experiments are the only way to determine what state the cell is in on its way to being differentiated . </P> <P> For a number of cell cleavages (the specific number depends on the type of organism) all the cells of an embryo will be morphologically and developmentally equivalent . This means, each cell has the same development potential and all cells are essentially interchangeable, thus establishing an equivalence group . The developmental equivalence of these cells is usually established via transplantation and cell ablation experiments . </P>

State the method of cell division used in embryonic development