Chromosomes at prometaphase and metaphase of the first cleavage in mouse and hamster eggs

The chromosome behavior from the late pronucleus to the first cleavage was examined in air‐dry preparations of mouse and hamster eggs. At recovery 88% of 327 eggs from 28 mice and 93% of 185 eggs from 13 hamsters were showing various stages during fertilization. The presence of fuchsin positive, intertwining and condensing chromatin fibers was first seen in the late pronuclei and later the fibers contracted into individual elements. In some pronuclei the number of chromatin fibers was greater than the haploid number for the species. The degree of contraction of the fibers was different in the male and female pronucleus of some eggs. Of 22 mouse and 12 hamster eggs at these stages, 18 (82%) and 6 (50%) respectively exhibited asynchronously contracted fibers. In eggs at the stage of “syngamy” the two groups of chromosomes were closely associated and intermingled. Of eight mouse and nine hamster eggs at “syngamy,” only one (13%) and O respectively were asynchronous in the degree of chromosomal contraction. The chromosomes at metaphase were in a single group and individual chromosomes with two chromatids were seen. At metaphase, 4 of 33 mouse eggs (12%) and 1 of 20 hamster eggs (5%) were asynchronous. Forty‐seven, 55 and 58% of mouse eggs were at “syngamy” or at later stages when recovered 22.5–23, 23.5, and 24–24.5 hours and 3, 26 and 34% of hamster eggs were at these stages when recovered 14–14.5, 15–15.5 and 16–17 hours after ovulation respectively. The results indicate that prometaphase of the first cleavage occurs over a longer range of time in mice than in hamsters.