THE PHYSIOLOGY OF THE EMBRYONIC MAMMALIAN HEART BEFORE CIRCULATION

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Abstract
The hearts of rat embryos were observed in hanging drop cultures. Of 527 embryos obtained from 53 rats 292 were placed in cultures, heart rates were counted on 138 and cinematographs made of 48. The left of 2 lateral heart primordia began contracting at 3 somites or 9 days and 14 hrs. Contraction progressively involved heart-forming splanchnic mesoderm until a single median ventricle was active at the time the 5th somite was formed. The contraction of the atrium was first seen in slow-motion cinematographs, before it was recognizable morphologically, as a small center of activity near the atrioventricular junction at the end of the 6 somite stage. The circulation of blood began at 8 somites. The heart rates at somite ages from 3-8 are tabulated. Evidence is presented that each chamber has the power of spontaneous rhythmical contraction at its intrinsic rate, that contraction progresses as a wave, and that the atrio-ventricular interval appears when the atrium begins contracting.