Figure 4.

per1 and clk1 are expressed ubiquitously in the zebrafish embryo. (A) Whole mount in situ of per1, and (B) clk1 transcripts in embryos reared in an LD cycle and fixed at ZT3 on day 2 display a gradient of ubiquitous transcription with the highest level of expression at the anterior. Sections through the head (C–F) show no discrete region of the brain or retina at this stage, which due to a distinctive expression of per1 could be identified as the location of a centralised pacemaker (arrowhead indicates location of pineal gland).