Orientation and behavior of epithelial cell muscle processes during Hydra budding.

The reorientation of the muscle processes of the ectodermal epithelial cells in Hydra attenuata has been examined during bud morphogenesis and in grafts inserted at right angles to their original orientation. The muscle processes were observed in histological preparations after staining with Mallory's Triple Stain and, in freshly fixed hydra, with polarization microscopy. Since the bud forms as an outpocketing from the parent, the ectodermal muscle processes, which run longitudinally, must reorient at some stage in order to be in the proper orientation on the bud. During the early stages of bud development, the reorientation of the ectodermal muscle processes is a passive result of the deformation of the parental tissue. Later, active reorientation occurs. The means by which muscle processes are able to reorient was studied further in rectangular grafts placed at right angles to their original orientation. In these grafts, muscle processes become aligned with those of the host in two ways: (1) in groups through the rotation of the whole graft or (2) individually, probably by retracting and then reextending in the new direction.