Fig. 4. Developmental profiles of neurones immunoreactive to RPCH and CCAP during embryonic development. (A) RPCH-IR neurones (X-organ) and immunopositive endings within the sinus gland (sg) in the eye, 5-6 days after eye anlage formation. (B) RPCH-IR structures in the brain and eye 6-7 days after eye anlage formation. Note that immunoreactivity in the X-organ (arrowheads) is weak, but that three pairs of prominent cells in the protocerebrum (arrows) now give rise to descending axons. (C) RPCH-IR structures in an embryo 2-3 days before hatching. Note descending axons, some (if not all) of which project contralaterally, anterior to the oesophagus (oe, arrow), and the prominence of, and connections to the putative post-commissural organs (pco), which dorsally project three digitate extensions (small arrows). (D) CCAP-IR structures associated with the thoracic ganglion and pericardial organs during mid-eye development. Large arrow, the posterior bar of the pericardial organ; small arrows, contralateral projections of axons reminiscent of those seen in the adult. Perikarya were never observed in these preparations. Abbreviations: oe, oesophagus; pco, post-commissural organs; s, segmental nerve; sa, sternal artery; sg, sinus gland. Scale bars: 50 µm.