888 RADIATION BIOLOGY 



kidneys, defects in the mesonephros, inhibition or duplication of a meta- 

 nephric bud, and epispadias. In the central nervous system, the brain 

 was the only part affected by irradiation on day 10; in all but one of the 

 cases, malformations involved the forebrain, mainly the telencephalon. 

 A tendency to reversed asymmetry in dominance of aortic arches, com- 

 bined in two of three cases with reversed tail curling, gave indications of 

 a certain degree of situs inversus. Abnormalities of the extremities were 

 usually more pronounced in, or limited to, the forelimbs. In mild cases 

 only the distal elements were affected, while distortions and deficiencies 

 extended more proximally in more severely afflicted individuals. 



Set apart from the malformations are cases of localized retardation, i.e., 

 tardiness in normal processes rather than aberrant growth. Criteria for 

 gauging retardation are considered not completely satisfactory, especially 

 where malformations are superimposed. Brain and urogenital organs 

 are most frequently retarded, but heart, aortic arches, and lungs may 

 also be affected. Also classed with retardation rather than malformation 

 is liver damage, which frequently involves a reduction (occasionally down 

 to complete absence) of hemopoietic elements. While the impairment is 

 only slight and transitory in the 100-r groups, the authors feel that in the 

 200-r groups it represents a significant functional loss and may be associ- 

 ated Avith the high rate of prenatal death. As judged by eight 100-r 

 animals allowed to go to term, there may be recovery from localized 

 retardations except in the eye, where microphthalmia may become 

 anophthalmia or malformation, and in the heart (one case). 



A few results of irradiation on day 9 are mentioned by Wilson and Karr 

 in an abstract (1950) and used (1951) for occasional comparisons with 

 the day-10 results. While reversal of symmetry was only indicated 

 following irradiation on day 10, treatment on the preceding day may 

 give situs inversus totalis. Furthermore, there seems to be a very sharp 

 division for the type of central nervous system anomalies obtainable from 

 the two treatment stages, day-10 irradiation never yielding the class of 

 defects that results from faulty closure of the neural tube, while irradia- 

 tion on day 9 often produced meningocele, encephalocele, cranioschisis, 

 etc. More prevalent after day-9 exposure were aberrant cell islands of 

 neural origin, growing independently in the mesenchyme surrounding the 

 brain. These are described in more detail by Wilson, Brent, and Jordan 

 (1951) and may be comparable to the regenerative growths found by 

 Kosaka (1928b, c, see p. 880). 



In one experimental series, Russell et al. (1951) concentrated on the 

 irradiation of day 11^. However, since this experiment also involved 

 irradiation under hypoxia, it will be reviewed in a different section (see 

 Sect. IIIB). 



The experiments of Raynaud and Frilley fall into a class by themselves 

 since, because of the method employed, the results cannot be used to 



