Broiler performance is known to be related to embryonic developmental parameters. However, strain or genotype differences with regard to embryo physiological parameters and juvenile growth have received little attention. A total of 1,200 hatching eggs produced by Cobb and Ross broiler breeders of the same age were studied. At setting for incubation and between 66 and 130 h of incubation, egg resonant frequency (RF) was measured as an indicator of embryonic development. Also, eggs were weighed before setting and at d 18. From d 10 to 18 of incubation, remaining albumen was weighed. During the last days of incubation, hatching events such as internal pipping (IP), external pipping, and hatch were monitored every 2 h. Hatched chicks were recorded and weighed. At IP stage, gas partial pressures in the egg air chamber were measured. Hatched chicks were reared for 7 d and weighed. Results indicate that RF of Ross eggs were lower than those of Cobb eggs (P < 0.01) and starting time point of RF decrease occurred earlier in Cobb eggs than in Ross eggs. Relative egg weight loss up to 18 d of incubation was lower in Cobb than in Ross (P < 0.05). At IP, partial pressure of CO2 was higher in Cobb than in Ross (P < 0.05) with shorter incubation duration in Cobb. Between 6 and 60 h posthatch, heat production was higher in Cobb than in Ross (P < 0.05). At 7 d posthatch, Cobb chicks were heavier than Ross chicks (P < 0.05). It is concluded that Cobb and Ross embryos-chicks have different growth trajectories leading in different patterns of growth resulting from differences in physiological parameters.