The development of the post-naupliar germ band of Gammarus pulex is described from early germ disc to segmentation and limb-bud formation. After gastrulation, at the hind edge of the blastopore, scattered ectoderm cells begin to arrange themselves in regular rows in antero-posterior and mediolateral directions. Ectoteloblasts are not differentiated. The meristematic zone producing the material for the rows shows only irregular divisions. In all, 17 transverse ectoderm rows are formed in this way. Rows E(2) to E(17) produce four regular descendant rows each cleaving twice following the mode of a mediolateral mitotic wave. Thereafter the resulting grid-like pattern is dissolved by defined differential cleavages that are traceable until the third differential cleavage. During the differential cleavages, neuroblasts are formed. The first postnaupliar ectoderm row, E(1), differentiates in a different manner. Posterior to the blastopore region, eight mesoteloblasts are differentiated by a complicated pattern of cleavage at the same time as the beginning of row formation in the ectoderm. During mesoteloblast differentiation, two small mesoderm cells on each side are formed underlying ectoderm row E(2) and three pairs of mesoderm cells belonging to ectoderm row E(3). After differentiation the mesoteloblasts give rise to 14 complete (eight-celled) mesoderm rows corresponding to ectoderm rows E(4)-E(17). The cells of the mesoderm rows undergo a specific set of divisions. The mesoderm row corresponding to ectoderm row E(2) gives rise to a cell migrating backwards to the area of row E(3). Mesodermal and ectodermal events are analysed in relation to segmentation. The intersegmental furrows become visible after the second cleavage of an ectoderm row. The corresponding mesoderm row has to be complete and is positioned under the two posterior descendant rows of an ectoderm row. Cytological changes of the ectoderm cells are recognizable from the beginning of segmentation. Bulging of the limb bud is related to the first cleavage of a mesoderm row and the second differential cleavage of an ectoderm row. Each segment is composed of derivatives of two ectoderm rows. Segmental boundaries lie within the area of the descendants of one ectoderm row and do not correspond to genealogical borders. The mesoderm of a segment arises from one mesoderm row, except in the first thoracic segment where two mesoderm rows are involved.