1. Williams, W. L., Veterinary Obstetrics, second ed. (published by the author, Ithaca, New York, 1931).
2. Haselhorst, G., Z. Geburtsh. und. Gynäk., 95, 400 (1929).
3. Schmitt, W., Z. Geburtsh. und Gynäk., 90, 559 (1926/27).
4. Panigel, M., J. Physiologie, 51, 941 (1959).