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Location of the muscles of the chick embryo that were responsive to ATP. Three chick embryos from stages 35–37 were killed, and each muscle was identified and tested in at least two of the three embryos. All muscles tested in embryos of these ages contracted in response to ATP. By embryo day 17 (stage 43), none of the muscles contracted in response to ATP (reproduced from [79] with permission from Wiley-Liss, Inc.)
