DEVELOPMENTAL BIOLOGY, 7, 605-616 (1963) Chromosomal Aberrations in Embryos from Overripe Eggs1 EMIL WITSCHI~ AND RUBEN LAGUENS” Department of Zoology, State University of lowa, Iowa City, Iowa Accepted November 16, 1962 INTRODUCTION Ovulated frog eggs stored in uteri beyond a normal length of time before fertilization suffer degenerative changes which finally result in the complete loss of developmental capacity and death. The physi- ologist Pfltiger (1882) introduced the term overripeness for this condi- tion. He observed that from moderately overripe batches of eggs, more than the normal 50% of male offspring is obtained. Later in- vestigators found that overripeness causes equally or even more striking deviations also from normal somatic development (literature in Witschi, 1952). The morphologic and physiologic analysis of over- ripeness was started by Witschi and his collaborators but still awaits much further work. In true frogs of the genus Rana, uterine eggs are at the stage of the second meiotic spindle at metaphase. The cast-off first polar body usually remains undivided. This is a stage of arrest which under favorable temperature conditions (9’C in Rana temporuriu) lasts from a few hours to 1 day, but may be prolonged by intervening cold weather spells for days without untoward effects. Overripeness de- velops mainly in laboratory frogs that are kept at room temperatures (over 15°C). In Xenopus another type of overripeness was recently described by ’ Supported by grants by the National Science Foundation and the National Institutes of Health. The authors wish to express thanks also to Dr. Kazuya Mikamo and Dr. D. C. Johnson for assistance in the preparation of this work. ‘Present address: Institute of Anatomy, Medical Faculty, University of Basel, Basel, Switzerland. ‘Present address: Facultad de Ciencias Medicas, Universidad National de La Plata, La Plata, Argentina. 605