Fig. 1

Overview of pharyngeal segmentation in Chick (Gallus gallus) and catshark (Scyliorhinus canicula) embryos. Longitudinal sections through the pharyngeal region of whole mount anti-laminin immunostained chick (a, b) and catshark (c, d) embryos during the period of segmentation. a Section through a stage 14 chick embryo, at which point the first three pharyngeal pouches have formed. The contact between the pouches and the endoderm can be clearly seen. b Section through a stage 19 embryo, by which time all four pharyngeal pouches have formed. It is now quite clear that the different pouches have assumed different morphologies. The first pouch has receded from the ectoderm and mesenchyme lies between, while the second pouch has broken through to create an external opening. The interface between pouch 3 and the ectoderm is thinning at this point while pouch 4 is still abutting the ectoderm. c Section through a stage 19 catshark embryo, at which point the first three pouches have formed and are in contact with the ectoderm. d Section through a stage 22 catshark embryo which has formed 5 pouches. At this stage pouches 1 to 4 have broken through externally while the fifth pouch is just beginning to do so