A series of morphogenetic movements separate and rearrange the cells of the blastula to form the three primary germ layers of the gastrula. The cells and the extracellular matrices involved in these movements will be examined to determine how the cell surface participates in morphogenetic movements. There are five specific aims. I. There are three cell surface changes that occur as the primary mesenchyme cells ingress from the wall of the blastula of the sea urchin embryo. The fist goal will be continue efforts to characterize the molecular basis of those changes. II. Following ingression primary mesenchyme cells begin a series of characteristic cell movements on the inner wall of the blastocoel. By microinjection of cells, experiments will address the cellular and the substrate changes that participate in the behavioral changes. The molecular basis of those changes will be examined with monoclonal antibodies using a functional screen that was developed for determining adhesive function of cell membrane antigens. Confirmation of function will involve microinjection of antibody into the blastocoel to block specific movements by the primary mesenchyme cells. III. Six antigens have been detected that appear to be involved in the adhesion of cells during gastrulation. One antigen is specific for ectoderm, four of the others are specific for endodermal cells. These will continue to be characterized. IV. The basal lamina has been studied and found to be essential for invagination of the archenteron. Experiments are proposed to better establish the relationship between the basal lamina and the presumptive endoderm that is required for expression of endoderm-specific proteins and subsequent invagination. V. A number of cell lineages have been defined by the monoclonal antibodies used in these studies. These lineages will be studied further in order to establish the range of patterns present on the surface of embryonic cells that could be used as morphogenetic markers.