Weak allele. Embryos derived from fu¹/fu^10C mothers have a weak phenotype. Additional rows of denticles with reversed polarity are seen behind the wild-type denticle belts. At least one third of homozygous fu¹ embryos derived from fu¹/fu^10C mothers do not develop; some are not fertilised and some stop cleavage after a few cell divisions. The remainder form a normal cellular blastoderm which may be reduced in size and contain very little yolk compared to wild-type. These embryos generally gastrulate normally. At approximately 5.5 hours of development, cells along the germ band appear loose and dead cells can be seen first in the cephalic mesoderm and then in the posterior mesodermal regions. Extensive cell death is seen in the mesoderm and ectoderm of 7-8 hour embryos, but not in the endodermal derivatives, which develop normally during this period. During germ band shortening, occasional pairwise fusion of segments can be seen, especially in the more posterior segments, and the ventral cord is often discontinuous. Development of the embryo from 9.5-20 hours is relatively normal.