Abstract
ABSTRACTA standardized morphological time scale for prenatal mice is presented, which is useful from 13·0 to 17·0 days chronological age with an accuracy of 0·1 day. Morphological age for an embryo or fetus is shown to correlate highly with ages estimated from body weight and crown-rump length. The time scale is used to study the comparative development from 14 to 17 days prenatal age of an F2 and four inbred mouse strains. The F2 mice average 0-5 day ahead of C57BL/6 mice, and C57BL/6 mice average 0·5 day ahead of A, BALB/c, and, at some ages, DBA/2 mice. Using reciprocal F1 crosses and reciprocal backcrosses, it is also shown that both the fetal heredity and the maternal environment contribute significantly to the more advanced development of hybrid mice.
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