Fig. 2. (A-O) Expression pattern of the neurogenic genes in the prosoma. Whole-mount in situ hybridisation; anterior is towards the top. (A-E) CsDelta1 expression; (F-J) CsDelta2 expression; (K-O) CsNotch expression. (A,D) At 130 hours, CsDelta1 expression is visible in the first five to eight invaginating cell groups that have formed per hemisegment (arrows). (B) Seventeen to 20 additional invagination sites (arrow) were generated at 170 hours, all of them expressing CsDelta1. (C) When the apical cell processes of the invaginating cells lose contact to the apical surface at about 190 hours in the prosoma, CsDelta1 expression decreases, although some of the invaginating cell groups still show a strong expression (arrow). (E) At 180 hours, the final number of 30 to 32 invagination sites is visible. CsDelta1 transcripts can be detected in all invaginating cell groups (arrow). (F,I) CsDelta2 is uniformly expressed in the ventral neuroectoderm before formation of invagination sites (data not shown). At 130 hours, CsDelta2 shows stronger expression in the invaginating cell groups (arrows), although it is still expressed in all neuroectodermal cells. (G,J) The upregulation of CsDelta2 expression is also visible in subsequent invagination groups. (H) Expression of CsDelta2 decreases at the same time (about 190 hours) in the invaginating cells as CsDelta1 expression. (K,N) CsNotch is expressed in the whole ventral neuroectoderm at 130 hours, but shows a slightly stronger expression in the lateral regions of the hemisegments (arrows). (L,O) CsNotch is still expressed in all neuroectodermal cells at 170 and 180 hours, but there is heterogeneity in the expression levels (arrows). (M) The same distribution of CsNotch transcripts is visible at about 190 hours, but in addition a medial group of cells shows strong CsNotch expression (arrow). 11 to 14, walking legs 1 to 4 (corresponding to prosomal segments 3 to 6). Scale bars: in A, 200 µm for A-C,F-H,K-M; in D, 25 µm for D,E,I,J; in N, 50 µm N,O.

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