SummaryPreimplantation embryo development was studied in the golden hamster (Mesocricetus auratus). Three groups of regularly cycling female hamsters were used: (I) 30 nulliparous young female (NYF) hamsters; (II) 24 nulliparous adult female (NAF) hamsters and (III) 30 multiparous adult female (MAF) hamsters. Female hamsters were mated with male hamsters of proven fertility. Only 15 min were allowed for mating. The moment of ejaculation was registered. Female hamsters were killed from 60 to 69 h after coitus. Corpora lutea were counted in both ovarian surfaces. Oviducts and uterine horns were flushed separately and embryo number, stage of development and distribution were recorded. Adult female hamsters, nulliparous and multiparous, had significant higher ovulation rates than NYF, but their reproductive efficiency was significantly lower. Preimplantation embryo development and transport were highly synchronous in NYF, but not in adults. Morulae were observed in NYF as early as 62-63 h after coitus. In adult female hamsters, significant numbers of morulae were found until 66-67 h. On the contrary, in NYF four-cell embryos were detected only until 60-61 h, while four-cell embryos were found until 64-65 h in NAF, and until 66-67 h in MAF. Embryo transport from the oviduct to the uterus is practically completed at 62-63 h after coitus in NYF, while it is evidently retarded in adult animals. In NYF all eight-cell embryos reached the uterus by 62 h after coitus. In adult female hamsters, both nulliparous and multiparous, a considerable number of eight-cell embryos fail to migrate into the uterus even at 67 h after coitus.

The contamination levels of Toxocara spp. eggs in soil samples from a university campus in Mexico City were evaluated and analysed according to garden size, and were related with the percentage of Toxocara spp. eggs and its viability according to the soil characteristics. A total of 1458 soil samples collected in 15 gardens (six large and nine small) were analysed by sedimentation-flotation with zinc sulphate solution on at 33%. Contamination was low (12.9%), and egg viability was high (65.5%). The size of the garden had no influence on the presence and viability of Toxocara spp. eggs. Contamination was negatively correlated with the percentage of vegetation (r = -0. 61, P < 0.01) and the viability was negatively associated with the percentage of clay in the soil samples (r = -0.51, P < 0.04). The size of the garden did not influence the presence and viability of Toxocara spp. eggs.