A total of 39,280 eggs obtained from two selected White Leghorn strains over 4 generations representing 290 sires were incubated. 2. The unhatched eggs were broken open for examination of embryonic developments and classified as early-dead (0-11 d), late-dead (12-22 d) and pips. 3. The sire-component heritability estimates (%) pooled over years in the two strains, respectively were 8.5 +/- 1.5 and 7.6 +/- 1.4 for total mortality; 0.5 +/- 0.5 and 6.3 +/- 1.2 for early-dead; 5.8 +/- 1.2 and 5.5 +/- 1.1 for late-dead and 5.0 +/- 1.1 and 3.1 +/- 0.9 for pips.
