Weight and vaginal opening in the albino rat

The body weight of rats at spontaneous vaginal opening (Wvo) was related to the weight at weaning (Ww) and the time elapsed between the two events (tvo) by the following equation: Wvo = aj + bWw + ctvo + residual where aj is the litter effect present when both the male and female young are reared together. When only female young are raised, aj may be replaced by a, so that in this instance there is no litter effect. Further model differences were noted for young raised under different conditions.