Early Embryonal Growth in Inbred Lines of Brown Leghorns and Their Crosses

FROM previous investigations (Blyth, 1952) it is evident that hybrid vigour in crosses between highly inbred lines of Brown Leghorns becomes apparent soon after hatching. However, the environment of the hybrid embryo (egg content or composition) does not permit heterosis to be manifested before hatching. This may be due to the restriction by the total food supply of the egg, which might not be expected to limit early growth. Such a relationship was found by Copeland (1940) in plants where hybrid embryos show heterosis during very early growth, whereas they do not differ from inbreds at the time of seed maturation. There is no agreement in the literature concerning the role of genetical and environmental influences on the early growth of chicken embryos. Byerly (1932) assumes that developmental rate of chicken embryos is at first approximately identical in different breeds, and changes only later on in direct proportion to the …