2.2. In situ hybridization

We performed conventional in situ hybridization (ISH) on tissue sections as described in a previously published protocol [17], except that we omitted the proteinase K treatment and the subsequent fixation steps were omitted. We used partial or full-length clones as riboprobes, which we obtained by reverse transcription–PCR, using the primers listed in the electronic supplementary material, and sequenced. The gecko-chordin probe was kindly gifted by Dr H. Kiyonari (Riken, Japan) [18].