Oligosyndactylism, an early embryonic lethal in the mouse.

The stock of Os mice studied was maintained in the Department of Animal Genetics, University College London. During the later part of the work, a colony was established at New York University Medical Center from mice kindly sent by Professor Griineberg. Embryos from Os/+ xOs/ + and from Osj + x +1+ matings were examined between the 3rd and 8th day of gestation. Age was determined by the standard vaginal plug method. The morning on which a vaginal plug was found was taken as day 0. Capsules containing 7and 8-day-old embryos were removed from the uterus before fixation. In 5and 6-day pregnancies the entire uterus was fixed, but only the regions of decidual swelling were sectioned. There was good agreement between the number of implantations and the number of recent corpora lutea. In the studies of 3and 4-day-old embryos the ovaries, oviducts and uterus were sectioned since there was no external evidence of the location of the embryos. The ovaries were examined immediately for young corpora lutea, which, unlike old ones, are a rosy pink before fixation, to determine if these females were pregnant. This procedure eliminated the time-consuming preparation of late pseudo-pregnant uteri. Most of the tissues were fixed in Bouin's solution, embedded in paraffin wax, serially sectioned at 7, 8 or 10 /i, and stained with Ehrlich's hematoxylin and eosin. A number of embryos 6 days and older were fixed in Carnoy's fluid