Regionalisation of segment-forming capacities during early embryogenesis inDrosophila melanogaster.

Transverse fragmentation of the egg ofDrosophila melanogaster results in the formation of partial larvae. Anterior and posterior egg fragments develop the respective partial larval patterns. The partial patterns do not add up to the complete pattern.Fragmentation near the middle of the egg during early cleavage causes a gap of 3-4 segments on average in the larva. This gap is reduced to 2 segments on average if operations are performed at the early syncytial blastoderm stage. Fragmentation near the pole regions from early cleavage stages onwards causes a gap of only 2 larval segments on average. When the egg is fragmented at the columnar cellular blastoderm stage or later, the gap at all positions amounts to the size of one segment or less. A gap is also found after incomplete fragmentation, when the ooplasmic bridge between both egg parts was constricted beyond a certain limit.A specific shift of the segment-forming capacities along the egg axis is observed from syncytial blastoderm stages onwards.After partial longitudinal fragmentation no additional structures are observed. In general, the partial transverse patterns add up to the complete pattern, but minor structures like single denticles are missing near the fragmentation site.The results are discussed with respect to current concepts of segment pattern formation during early embryogenesis in dipterans.