Recent decades were characterized by genetic selection of broiler and layer chickens for enhanced growth rate and meat yield or intensified egg production, respectively. It is to be expected that genetic selection for various traits would also influence embryo development and growth patterns that affect metabolism. The objective of the present study was to examine the effects of broiler (Cobb and Ross) and layer (Lohmann) lines and parent flock age (31 and 38 wk) on embryonic development, heart rate, O2 consumption, and blood parameters. For each line, 2 incubation sets, from flocks aged 31 and 38 wk, with 500 eggs per set, were studied. Development patterns differed between layers and broilers: layers hatched 1 d later and their relative embryonic weight at hatch was significantly lower, probably because of their longer period until hatch, although yolk relative weights were similar. Oxygen consumption of layer embryos was lower than that of broilers, and plasma triiodothyronine concentration, hematocrit, and hemoglobin levels were lower in layers than in broilers. However, layer embryo heart rate was higher from embryonic d (E) 15 onward. Differences were found between the Ross and Cobb lines in embryonic development. Oxygen consumption of Ross embryos was slightly higher than that of Cobb from E16 to E19. Heart rate of Ross embryos was significantly higher than that of Cobb. Furthermore, plasma triiodothyronine concentration of Ross embryos was significantly higher on E14, E16, and hatch. These differences suggest that the genetic selection for rapid growth rate in the 2 broiler lines did not cause differences between their embryonic growth patterns, but it did affect their metabolic rate. Oxygen consumption was higher in embryos from the 38-wk-old flock. The results suggest that genetic selection affected not only production traits but also the developmental pattern of the embryo and its metabolic characteristics.