Disaggregated single cells from the embryos of Strongylocentrotus purpuratus were allowed to reaggregate and reform quasi-normal embryos in culture. In this investigation, emphasis was placed on cell migration and its role during cell segregation. Epithelial cell migration appears to be initiated through the sequential formation of (a) cytoplasmic blebs, (b) microvilli, (c) filopodia and (d) hyaloplasmic lamellae. After functioning as a locomotive structure during cell migration, the lamella enlarges and forms a hyaloplasmic sheet, which makes up part of the blastocoel wall. The blastocoel wall is the first recognizable structure reformed in the reassembling embryo.

Fibroblasts from 5½-day-old chick embryos go through a sequential series of changes when exposed to a constant electromagnetic field (EMF) of 10 V/cm. These changes include rounding up, becoming bipolar in shape, assuming a cylindrical profile, elongating perpendicular to the EMF, and migrating to the cathode. These morphological changes are associated with changes of the cell surface, which include the formation of filopodia and extensive sheet-like contacts on the cathodic cell surface, the retraction of processes and the formation of focal contacts on the anodic cell surface.

Time-lapse and electron microscopic observations were made on both epithelial and mesenchymal cells during the reassembly of embryos from dissociated cells of Strongylocentrotus purpuratus. In epithelial cells, where lysosomes are produced through the fusion of saccules formed from Golgi bodies, both phagocytosis of cell debris and resorption of differentiated cell structures were observed. In these cells, the lysosomes migrate and fuse with both autosomes and phagosomes. On the other hand, in the mesenchyme cells, where lysosomes are produced through the direct enlargement of the Golgi body’s cisterna, neither phagocytosis nor resorption was observed. The migration of the lysosomes to the epithelial cell margins is the first indication of a re-establishment of cellular polarity after dissociation.

Disaggregated singie cells from the gastrula of Strongylocentrotuspurpuratus were studied as they reaggregated and reformed quasi-normal embryos. In this investigation emphasis was placed on the structural events involved during the reformation of cell contacts vis-a-vis cell migration. The early cell contacts are non-junctional cell appositions, which are characterized by non-parallel apposing membranes. Between post-migratory epithelial cells, there is a shift from non-parallel to parallel apposing membranes. These cell appositions are found between the overlapping lamellapodia along the apical margins of the epithelial cells during blastocoel enlargement. Incipient continuous junctions are formed by the deposition of an electron dense material in the intermembrane space. As the junction develops, electron dense plaques form in the cytoplasm immediately subjacent to the junction and septa form between the apposing membranes.