Published online by Cambridge University Press: 14 April 2009
Body and liver weight and the number of nuclei in the liver were significantly higher in F1 hybrid than in inbred embryos developing in the same uterus after artificial heterospermic insemination. The size of 13-day-old F1 hybrids corresponds to the size of 13·4-day in bred embryos. No significant differences in cell size were found between F1 hybrid and inbred embryos of the same age. It is concluded that in 13–15-day embryos heterosis takes place through increase in cell number and not cell size.