170 ORVILLE T. BAILEY 



fully grown ones, and the period between radiation and overt tissue degen- 

 eration is shortened (Clemente et al., 1960; Mogilnitzky and Podljaschuk, 

 1928, 1929; Scholz, 1934, 1935; Yamazaki ct al, 1960; Ziminern and 

 Chavany, 1931 ) . 



Clemente et al. (1960) found that as little as 125 r of roentgen radiation 

 to the head may result in microcephaly and cataracts if gi\en to rats 8 hours 

 old, while 300 r produces abnormal neurologic signs and histologic changes 

 in most rats at 1 day and 4 days of age, but not at 7 days. They feel that 

 resistance of the brain to radiation becomes significantly increased toward 

 the end of the 2nd week of postnatal life. Their histologic studies indicate a 

 high degree of correlation between abnormal neurologic signs and histologic 

 lesions in these immature mice. While some changes, predominantly vascular, 

 are found in rats sacrificed at 48 72 hours, they state, "It seemed as though 

 processes were under way which would result in larger necrotic sites, espe- 

 cially in rats administered 1,000 r, had these animals been allowed to live 

 for longer periods. This assumption seems especially \alid since other animals 

 radiated at the same postnatal time and sacrificed 1 to 14 months later 

 showed larger lesions in the brain." (Clemente ct al., 1960). 



This work is in some way reminiscent of Hicks's ( 1953, 1954) results in 

 antenatal development. The occiurence of cerebellar changes and micro- 

 cephaly is in accord with his timetable. The eye defects are of a different 

 type from those Hicks produced by radiation early in pregnancy. 



For this reason, the results of Clemente it al. (1960: Yamazaki et al., 

 1960) in some ways may correspond to late prenatal radiation in certain 

 other species of animals in which brain development is more advanced at 

 birth. However, they are of particular interest because they are quantitative 

 studies in brains with no, or restricted, regenerative capacity. They are also 

 in accord with results in puppies reported by others (Lyman et al., 1933; 

 Scholz, 1934,1935). 



It seems reasonable to conclude that the nervous system of young animals 

 is considerably more sensitive to radiation than that of adult animals of the 

 same species, that the time inter\al between radiation and o\ert histologic 

 evidence of degenerative changes is less in young animals, and that abnormal 

 neurologic signs, behavioral changes, and histologic lesions are regularly 

 produced with lower dosages of radiation in young animals than in old 

 ones. The particular alterations that accompany maturation in the neuron 

 so that it changes its response to ionizing radiation in these ways remains an 

 important problem. 



Vascular Responses to Radiation 



One characteristic of the reaction to radiation in all tissues is the develop- 

 ment of degenerative and occlusive changes in blood \essels. The central 



