Pathways of Radiobiological Damage 261 



By appropriate matings, individuals could be obtained 

 derived either from irradiated cytoplasm and irradiated 

 (female) nucleus or irradiated cytoplasm and unirradiated 

 (male) nucleus. The dose required to inhibit development of 

 the latter was 54,000 rads., which was twenty-two times as 

 great as that required to inhibit the former. The author con- 

 cluded that in those animals which failed to develop after 

 irradiation of the cytoplasm only, the injured cytoplasm 

 acts in a direct manner in killing the embryo and not indirectly 

 through injury to the untreated chromosomes. However, 

 Nakao (1953) has recently presented evidence to show that 

 when fairly heavily irradiated silkworm eggs are fertilized by 

 unirradiated sperm within 2^ hours of irradiation, the eggs 

 which are laid show plenotypic changes which are character- 

 istic of the loss or mutation of certain genes located on the 

 paternal chromosomes. 



One of the earliest observable effects of radiation in many 

 types of cell is what is commonly referred to as the inhibition 

 of mitosis or of cell division. In eggs it is observed as a delay 

 in first cleavage, in yeast as a delay in the second budding 

 after irradiation, and in dividing tissues as an immediate fall 

 in the mitotic index. Most, or all, cells of a population which 

 is at a uniform stage of development are affected, and to an 

 extent which increases with the dose. It is measurable in grass- 

 hopper neuroblasts after a dose of only 4 r, in many plant 

 and animal dividing tissues after doses of about 50 r, in yeast 

 after about 1,000 r and in sea urchin eggs after about 10,000 r. 



Careful investigation of the phenomenon in several of the 

 classes of cell mentioned has revealed, as a common feature, a 

 prolongation of the time taken by the cell to pass from the 

 terminal stage of interphase to the end of prophase. The classi- 

 cal experiments of Henshaw (1940) on cleavage delay in the 

 eggs of the sea urchin (Arbacia punctilata), discussed quantita- 

 tively in detail by Lea (1946), have provided the following 

 information : 



(a) Irradiation at the dose levels employed does not affect 

 fertilization, the approach of the two pronuclei, or 



