The growth of the embryonic rat myocardium (Carnegie stages 13 to 23).

The relative growth of the myocardium was studied in the postsomite embryos (Carnegie stages 13 to 23) of 54 rats (Rattus norvegicus). Cavalieri's principle was applied to determine the myocardial volume of each rat heart. The calculated volume was related to the corresponding body weight and crown-rump length using a bivariate allometric equation. The resulting scattergram proved to be discontinuous: the rate of myocardial growth in the postsomite embryo was represented by two distinct phases. The first, from stages 13 to 20, was allometrically-negative; the second, from stages 21 to 23, allometrically-positive. This difference in myocardial growth rate might have been due in part to the functional increase of circulatory flow in the peripheral vascular bed in response to the demand for increased cardiac output that occurs at the conclusion of the embryonic period.