EMBRYONIC DEVELOPMENT 401 



more extensive and more varied abnormalities than did the injection of 

 radioactive physiological salt solution. These abnormalities became 

 apparent sometime after the birth of the individual. Vascular disturb- 

 ances, such as heretofore observed, were frequent and, in some cases, were 

 so severe as to cause the death of the animal. Moreover, other abnormal- 

 ities were often present such as gross imperfections in eye development, 

 arrested development of certain brain regions, particularly the cerebral 

 cortex, and in some cases arrested development of testes and ovaries. 

 Bagg's conclusion with reference to mammals, that there is a "marked 

 selective action of radium emanation on fast-growing embryonic struc- 

 tures, " is in accordance %vith the many observations on lower animals, 

 some of which have been described in preceding pages. 



In his later studies Bagg used X-rays instead of radium. By the use 

 of X-rays he obtained disturbances in the development of the vascular, 

 nervous and reproductive systems similar to those which resulted from 

 the use of radium, and, in addition, he found serious abnormalities in the 

 development of the kidneys (metanephroi). These kidney abnormalities 

 were principally the absence either of one or of both kidneys, or the 

 development of kidneys which were functionally imperfect. An embryo- 

 logical study of the kidney abnormalities found in the offspring of Bagg's 

 radiated animals has been made by Brown (26). 



Although the work of Bagg and his associates appears to establish 

 the fact that radiation of pregnant mammals causes serious abnormalities 

 in the offspring, conflicting reports are to be found in the literature. For 

 example, Levine (62) reported that the administration of relatively large 

 sterilizing doses of X-rays to white mice during pregnancy brought about 

 no defects in the offspring. An examination of the literature shows, 

 however, that reports such as that of Levine are in the minority. 



An extensive study of the possible inheritance of some of the develop- 

 mental defects of radiated mice and rats, particularly defects of the eyes 

 and the kidneys, has been made by Bagg and Little (13), Little and Bagg 

 (64), and Bagg (12). Literesting and important as these genetical 

 studies are, they do not properly fall within the bounds of the present 

 discussion. 



DIFFERENTIAL SENSITIVITY IN EMBRYONIC DEVELOPMENT 



One of the central problems of the effects of radiation on embryonic 

 development is that of differential sensitivity. With regard to this prob- 

 lem several questions arise. At what period during embryonic develop- 

 ment is an organism most sensitive to radium and X-rays? Does 

 sensitivity change with age? When are developmental abnormalities 

 most likely to occur? Which organs or organ systems of the embryo are 

 subject to the greatest alteration? The last two questions have already 

 been dealt with in foregoing pages. Sufl&ce it to say here, that the greatest 



