880 RADIATION BIOLOGY 



deuce for critical periods, tabular comparisons between experiments will 

 be made in an attempt to derive some general conclusions about certain of 

 the abnormalities. Many others will have to be ignored, due to limita- 

 tions in scope of this review. 



a. Experiments That Compare Results of Irradiating Different Stages. 

 The first investigator to attempt a survey of the gestation period with 

 regard to radiosensitivity was Kosaka who worked on mice (1927, 1928e), 

 rats (1928b), rabbits (1928a), 4 and guinea pigs (1928c), irradiating during 

 known intervals (though not definite days) postconception with various 

 doses, ranging from \£ to 2 SED, and observing effects manifest 6, 12, 24, 

 48, 72, et seq., hours after irradiation. A large part of the work on all 

 animals was devoted to histological description of tissue damage which 

 was apparently not considered as malformation since special mention is 

 made (1928d) of the fact that malformations (microcephalus, deformity 

 of the extremities) occurred only in the rabbit. For any given stage of 

 irradiation, tissues were ranked in order of decreasing sensitivity, and this 

 rank order was found to change with the stage in a manner parallel to the 

 change in relative growth rates of the particular organs. Early in the 

 period of organogenesis, brain and spinal cord are most sensitive, retina 

 and mesoderm in second place. Shortly thereafter, spinal cord loses 

 much of its sensitivity, while retina joins brain in first place. During 

 the period of the fetus, thymus suddenly becomes extremely sensitive, and 

 liver and spleen move up on the list. A variety of other tissues in the 

 lower ranks of sensitivity must be omitted from mention here. Finally, 

 certain organs showed no marked effects from irradiation at any stage. 

 One interesting finding was that, while the processes of regeneration in 

 most tissues were similar in kind to the growth which had occurred just 

 before irradiation, cerebrum and retina responded to severe damage by 

 forming numerous ependymal canals believed analogous to the neural 

 tube of early stages. 



Job et al. (1935) posed the question of whether certain periods in 

 development could be demonstrated to be critical either for the rat 

 embryo as a whole or for certain of its organ systems or individual 

 organs. After eliminating higher doses because of excessive mortality, 

 they obtained litters from sixty-six females which had received a single 

 dose of X rays of 90 r or less between the first and sixteenth days of 

 gestation. In all cases where irradiation had been before the eighth day 

 or after the eleventh day, the young were normal. Following irradiation 

 between eighth and eleventh days, 17 of the young (7 per cent) were 

 hydrocephalic, 14 (6 per cent) suffered from jaw abnormalities, and 52 



4 Unlike Kosaka's other publications, the paper on rabbits lacks an English or 

 German summary. This reviewer did not obtain a translation of the Japanese text. 

 Scattered information about the rabbit results was obtained from the other papers of 

 the series, particularly the summary paper (Kosaka, 1928d). 



