XIV. X RAYS AND X IRRADIATION 483 



creases. The cells commence vacuolization, and the mitochondria 

 break up. Chromosomes become fragmented or sticky and clump to 

 form pyknotic nuclei, and cell divisions do not occur. The pH of 

 the cell turns to the acid side, and death, as measured by further cell 

 divisions, occurs. 



Observations on X-ray effects on cells have been further refined 

 by using embryological material in the two cell stage and shielding 

 one of the cells from radiation, leaving the other exposed. These 

 techniques have sho^^^l that the nucleus and its contained chromo- 

 somes are the primary cell structures affected by the X rays. Critical 

 periods in the development have been established. Gastrulation is 

 one in which the organism is sensitive. Similarly, in organ develop- 

 ment, the blood-forming system, the nervous system, and the repro- 

 ductive system are highly susceptible to irradiation effects. On the 

 other hand, tissues that are not in division, as for instance, fat cells, 

 do not show pronounced changes. As development proceeds to the 

 adult, the effects of the radiant energy become less lethal to organ 

 systems with reduced cell division, i.e., the nervous system and the 

 skin. 



X-ray techniques ha\'e furthered cell lineage studies. Either the 

 sperm or egg may be irradiated to such an extent as to destroy the 

 nucleus but still permit normal fertilization and development to take 

 place. If the sperm receive the nucleus-destroying dose this may 

 lead to parthenogenetic development in which the female inheritance 

 contributes all the future characteristics to the individual. Similarly, 

 by irradiating the eggs before fertilization, fertilizing them, and then 

 allowing development to take place, it is possible to obtain individuals 

 that receive their inheritance from the male parent. 



Radiation combined with genetic methods offers further possi- 

 bilities. Cell lineage studies have been aided by use of an irradiated 

 embryo heterozygous for gene pairs affecting that tissue {22,46) . The 

 heterozygous individuals may be irradiated at different stages of the 

 embryological developmental period. The effects sought are those 

 destroying or mutating by X rays the dominant gene to the recessive 

 condition at some particular stage in development. When this oc- 

 curs, the cells derived from the mutated cell will express only the 

 recessive characteristics. The organ will become a mosaic of two types 

 of cells that may be differentiated by their phenotypes. By this 

 means it has been possible to time in development the stage at which 

 particular anlage are separated from tlic lest of the developmental 



