were noted but these signs appeared too mild to have contributed to death and were also seen in females that had no effect on life-span. The relatively low susceptibility of fetal mice to life-shortening was unexpected and could not be explained [U12]. 258. Friedberg et al. [F8] tested the effect of fast neutrons (15 rad) on mouse embryos in the pronuclear-zigote stage for their ability to shorten the life-span or to induce tumours in animals surviving at least 30 days after birth. No differences were found between irradiated and control animals of the same sex for the following end~points: mean ages at death, cumulative morta- lity distributions and incidences of the principal neoplastic diseases. 259. Sasaki et al. [S49] observed a significant shortening of life in mice irradiated at 16-18 days p.c. with 200 R of x rays. duced by about 13-16 per cent. Mean life-span was re- Slight changes of the tumour spectrum were ob- served, with no excess of lympho-reticular tissue tumours. Female mice had a higher incidence of lung and pituitary tumours, while a prevalence of lung and liver tumours was found in males. 260. In the rat there are data by Reincke et al. [R7]. In her experiments Wistar animals were administered a single whole-body x-ray exposure of 270 R at 5 days before birth and at 13, 49 and 121 days after birth. Among other ob- servations, life expectancy was reduced by about 3 to 6 months and this reduction was not influenced apparently by the age at exposure. In a subsequent paper Reincke et al. [R8] reported that 220 R 5 days before birth resulted in a long-term survival not significantly different from controls. 261. Sikov, Resta and Lofstrom [S31] studied the long-term mortality of rats surviving at the time of weaning exposures of 20 or 100 R at 10 days p.c. and of 50 or 185 R at 15 days p.c. life-span reduction appeared to be greater in females than in males where the effects observed were of doubtful significance. An interesting observation was that the LD . 50/30 of an irradiation performed at 100 days of age decreased as.a linear function of the dose of a previous irradiation carried out in the pre-natal period at all developmental ages. In the Opinion of the authors such a linear dependence would imply a general decrement of fitness as a result of pre-natal exposure.