LD50(24 HOURS) OBSERVATIONS IN THE CHICK EMBRYO AFTER EXPOSURE TO X-RAYS.

The chick embryo is recognized as an excellent tool for the study of radiation effects on embryonic tissues. The separation of the chick embryo from many maternal contributions permits evaluation of responses attributed solely to the effects of ionizing radiation on the embryo. The strict environmental controls that can be maintained during chick embryogenesis reduce to the minimum the variables that may alter the radiation response. As indicated previously by Boland (1) and by Goff (2), comparative evaluation of reported lethality studies on the chick embryo is difficult. Parameters have been varied by the investigators and include 112-kvp (3), 150to 180-kvp (4-8), 250-kvp (1, 2, 9, 10), and 1000-kvp X-rays (10), as well as Co60 (1, 11, 12) and radium y-rays (13) and fission neutrons (12). The dose rates in these studies varied from 5.3 r/min (13) through 220 r/min (7). Physical factors such as half-value layer were frequently varied, and the methods of dosage determinations were frequently unexplained, or the doses were not obtained at the embryo site. The postexposure study period varied from "shortly after exposure" (3) to LD5o0 (22 days), the lethality at the time of hatching (6). Some workers used nearly all ages (1 to 16 days) of embryos (2, 4, 6), whereas others used 20-hour (2), 33-hour and 60-hour (7), 50-hour (3), 4-day (8, 10, 12), or several ages (13, 14). As a result of the considerable variation in lethality studies reported by others, the estimation of LD50(24 hours) was undertaken to establish a reference point for a study of pathologic effects of ionizing radiation on embryonic tissues.