The position effect in mice on day 19.

This investigation on the position effect in mice on day 19 was based on a total of 610 fetuses from 51 litters of NMRI mice. The relative position of each fetus was determined by setting the distance from the center of each placenta to the cervix in relation to the total length of the uterine horn. After cessation of shrinkage due to fixation, the umbilical-cord and remaining membranes were removed and the fetuses were weighed to the nearest 0.1 mg. The variance for the weighing procedures was estimated at 4 mg. The fetuses were uniformly distributed on a segment beginning about 7% beyond the cervical end and proceeding to about 10% short of the ovarian end. A mixed model analysis of variance was applied to estimate the position effect: the deviation of the individual weights from the mean for each uterine horn were modeled as a second degree polynomial of the relative position. A position effect was found; the fetuses on the cervical end were lighter, those in the middle were roughly the same, and those of the ovarian end were heavier than the average.