Fig. 1. High magnification imaging of Ca_i²⁺ in early embryos. Ratiometric images of Fura-2-loaded HH4 or HH5 embryos. (A-C) Bright-field images at HH4 (A), HH4+ (B) and HH5- (C). (D-F) Fura-2 ratiometric imaging of embryos depicted in A-C. (Bottom row) Line-scan quantitation of the Fura-2 signal in embryos depicted in D-F plotted against right-left axis, at the position indicated by the white line in D-F. A-F are ventral views with anterior at the top; right (R) and left (L) are as indicated in A. (A,D) At HH4, a modest and symmetric Ca_i²⁺ enrichment appears at the anterior margin of the node. (B,E) At HH4+, Ca_i²⁺ enrichment expands posteriorly. (C,F) At HH5-, the prechordal plate splits the Fura-2 signal into distinct left and right Ca_i²⁺ fields; the left Ca_i²⁺ is more posterior and is enriched relative to the right. The blue spot in F is an artifact. ^*, Hensen's node; pc, prechordal plate; ps, primitive streak. Scale bar in A,D: 100 µm.