The dissertation consists of a series of experiments conducted to study developmental stability of various genetic stocks at different stages in the life cycle. The primary measures of stability were type and degree of asymmetry of bilateral traits and heterosis.

Higher relative asymmetry (RA), which was defined as (|L-R| / [(L+R)/2]) x 100, was observed in lines of White Leghorns selected 23 generations for high or low antibody response to sheep red blood cells than in their F1 crosses. The bilateral traits were 39-day shank length and length and weight of the first primary wing feather. Shank length was again measured on day 49 while body, heart, shank, and lung weights and ceca lengths were obtained on day 56. Heterosis was positive for organ sizes and negative for degree of RA.

Shank length and diameter, weight and length of the first primary wing feather, and distance between the junction of maxilla and mandibles and auditory canal (face length) were used to classify bilateral types and measure RA in six genetic stocks. The stocks were the S23 generation of White Leghorn lines selected for high or low antibody response to SRBC, sublines where selection had been relaxed for eight generations, and reciprocal crosses of the selected lines. Differences were found among all stocks for the traits measured. Rankings among traits for RA in descending order were face length, shank diameter, feather weight, and shank and feather lengths. The RA of shank and feather lengths did not differ from each other. The mean RA of the five traits was higher for the two selected lines than the crosses between them. The RAs of the two lines where selection had been relaxed was similar to that of selected lines.

In a line of White Rocks selected 39 generations for low eight-week body weight, bilateral traits measured were shank length and diameter, face length, and weight and length of the first primary wing feather of females at 240 days of age. The RAs of individuals that had not commenced egg production by 245 days of age were similar to those that had entered lay. In both cases, these RAs were higher than those of a subline in which selection had been relaxed for four generations.

Broiler sire lines had higher RA than dam lines for lung weight at hatch. Heterosis of RAs suggested superior homeostasis in F1 crosses than in the sire lines.

Based on populations studied, it may be concluded that RAs were trait specific with the RA of shank length being lower (0 < RA < 2%) than lung weight which was 10% or higher regardless of genetic background. The types of bilateral asymmetry exhibited although less consistent, still had consistency such that feather weight and ceca weight exhibited antisymmetry across different stocks. Length and width of shank and weight of lung, were generally of fluctuating asymmetry.

Heart:lung ratios differed among genetic stocks. In White Leghorns, lungs from late embryonic development to 25 days after hatch were heavier in a line which had heavier juvenile body weight than in one with lower juvenile body weight. In commercial broilers, heart:lung ratios at hatch were lower and thus inferior in parental lines than in their F1 crosses.