Fig. 1. The primary mesenchyme lineage of the sea urchin embryo and the expression pattern of VEGFR and VEGF during development. (A) Development of the primary mesenchyme lineage (red). The founder cells form at the 32-cell stage. At the mesenchyme blastula stage, the 32 descendants ingress into the blastocoel and become positioned along the ectoderm wall following a stereotypical pattern that includes two symmetrical ventrolateral clusters in which spiculogenesis begins by the formation of a tri-radiate rudiment. (B) Northern blot with total RNA isolated from embryos at the indicated stages. (C) Spatial distribution of VEGFR and VEGF transcripts detected by whole-mount in situ hybridization (WMISH). Embryos at the indicated stages were hybridized with sense (not shown) and antisense probes. Lateral views with the animal pole at the top, except for those marked (vv), which are ventral views with the oral side to the left. E, unfertilized egg; 16, 16-cell stage; 32, 32-cell stage; 60, 60-cell stage; EB, early blastula; B, blastula; SB, swimming blastula; MB, mesenchyme blastula; EG, early gastrula; G, gastrula; , prism; P, pluteus.