Figure 7.

(A-J): Proneural and neurogenic genes are re-expressed during formation of the secondary neural precursors. Flat preparations of the fourth and fifth prosomal hemi-segments after in situ hybridisation of whole embroys. (A-E) 220 hours of development, (F-J) 250 hours of development. Anterior is at the top, the midline to the left. (A) At 220 hours, CsASH1 expression has been down-regulated in all primary neural precursors (arrow) with the exception of one group (arrowhead). (B) At this time the pan-neural gene CsASH2 is still weakly expressed in the primary neural precursors (arrow). (C) CsDelta transcripts accumulate in the secondary invagination sites (arrow heads), while transcripts are down-regulated in the primary precursor groups. (D) A similar expression, although weaker, is visible after CsDelta2 in situ hybridisation. The arrow points to a region where CsDelta2 has been down-regulated, the arrowhead indicates expression in the secondary neural precursors. (E) CsNotch remains expressed at low levels in the ventral neuroectoderm. An up-regulation of CsNotch transcripts is visible in the secondary invagination groups (arrow). (F) At 250 hours CsASH1 expression can be detected in the secondary invagination sites (arrow), although it is not expressed in all of them. (G) CsASH2 seems to be expressed weakly in all secondary invaginating cells groups (arrow). (H) A high accumulation of CsDelta1 transcripts is visible in about 10 of the invagination sites (arrow), (I) while CsDelta2 seems to be xpressed in all invagination groups (arrow). (J) Cs Notch transcripts can be detected in all neuroectodermal cells at this time. l2 to l3, walking leg 2 to 3.