Changes in the concentration of calcitonin in the plasma of chick embryos during incubation

Calcitonin (CT) was assayed in the plasma of chick embryos at eight stages of development from day 17 of incubation to day 1 after hatching by a rat bioassay. The concentration of CT rose steadily from 1060 mu./1 on day 17 to 4700 mu./1 at the stage when pulmonary respiration was initiated on day 20. Nine hours later, when the embryos 'pipped' the shells, the concentration of the hormone more than doubled to reach a peak of 11000 mu./1. At hatching the level had fallen to 1400 mu./1 and 24 h later it was undetectable (less than 500 mu./1). The physiological significance of these changes is discussed in relation to the development of the embryo.