Fig. 1. pbl expression in the mesoderm is developmentally regulated. Whole-mount in situ hybridisation of wild-type embryos with a pbl RNA antisense probe. (A) During early stage 5, pbl transcripts are restricted to the pole cells. (B-F) Ventral view. (B) At late stage 5, pbl transcripts are observed on either side of the presumptive mesoderm. (C) At the onset of gastrulation, pbl transcripts are evident in the presumptive mesoderm (arrow). (D) This expression becomes restricted as stage 6 progresses (arrow). (E) pbl transcripts accumulate in a band of cells at the edge of the invaginating mesoderm (arrows). (F) As invagination proceeds, pbl transcripts are concentrated in the ventral furrow (arrow).