By direct ventricular puncture involving use of a micro-pipette and water manometer the ventricular blood pressures of 81 chick embryos ranging i n age from 2 to 7 days of age were obtained. The mean ventricular diastolic, systolic and mean blood pressures i n millimeter of water (given in the order) were as follows: 5 i 1.4; 21 -r-2.4 and 10 i-1.4 in two day hearts, 10 & 1.58; 27 i 6.68 and 14 ? I 4.32 i n three day hearts; 14 i 1.41; 39 & 6.22 and 19 2 2.39 i n four day hearts; 17 & 5.89; 63 5 15.49 and 32 2 12.08 in five day hearts; 5.45 ? 8.59; 74 i 17.94 and 34 2 16.76 in six day hearts. At seven days the mean ventricular diastolic pressure was 1.33 i 2.36 m m of water.
Ventricular diastolic pressure increased from the second through the fifth days, underwent a marked drop at six days and fell to near zero at seven days of development. Since in fully developed hearts ventricular diastolic pressure approaches zero if the valves of the outflow tract are competent, the following conclusion concerning valvular action was drawn; the valvular mechanism of the outflow tract in the developing embryonic chick heart is incompetent at two, three, four and five days of development.
It begins to become competent between the fifth and sixth days and attains competency at seven days.