Glass sponges begin as cellular embryos, like other metazoans, but after gastrulation the nuclei continue to divide without cells dividing, so that eventually a giant multinucleated cell is formed. There are some ‘cells’ that have single nuclei, but these are still connected to the multinucleated tissue by open or plugged cytoplasmic bridges. 
PictureEM of plugged junction
 
The plugs (shown in the EM image on the left) are an odd osmiophillic junction, called a ‘plugged junction’. Labelling the nuclei with a fluorescent stain (Hoechst) and the microtubules with an anti-body to tubulin (as above left and center) shows that the syncytial are extensive in adherent cultures (shown on right). Nuclei in living preparations move through the tissue on vast bundles of microtubules and leave a trail of light marking their path.
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To understand how the tissue can be made up of a giant cell, the tissue can be dissociated through fine mesh (Nitex for example) and allowed to come back together. Similar experiments with cellular sponges provided the first illustration of cell adhesion molecules that were species specific, and in many instances individual specific. The video below shows a time-lapse sequence of fusion of tissues that have been dissociated from Rhabdocalyptus dawsoni and poured into a Petri dish. The counter shows time.