262 L. H. Gray 



fusion, all of which proceed at the normal rate, but it 

 does result in a great prolongation of the first mitotic 

 prophase. 



(b) Irradiation of either sperm or egg alone before fertiliza- 

 tion produces a comparable delay in cleavage. 



(c) The injury to the egg is one which is repaired at a rate 

 of about 1 per cent /minute at 20-25° C and 0-3 per 

 cent /minute at 0° C. No detectable repair takes place 

 in the sperm. 



(d) Irradiation of enucleated eggs prior to fertilization 

 causes no cleavage delay, though the time taken in 

 cleavage is in this case much longer than in the cleavage 

 of a normal egg. 



(e) When the sperm and the egg are each irradiated prior to 

 fertilization, the injuries sustained by each exactly 

 summate in their effect on cleavage delay, if allowance 

 is made for repair. 



The conclusion of Henshaw and of Lea that in this case the 

 prolongation of prophase is the result of a reparable injury 

 sustained by nuclear material seems inescapable. In view of 

 the period of the cell cycle which is critical in this phenomenon, 

 it is suggested that the injury is one connected with the con- 

 densation of the chromosomes. The further conclusion that 

 the nuclear material is directly injured is much less well 

 founded. Direct injury of nuclear material seems probable 

 since the delay is the same whether it is the sperm or the egg 

 which is irradiated despite the enormous disparity in the 

 respective volumes of cytoplasm and because of the absence 

 of effect in enucleated eggs. However, the sperm is not devoid 

 of cytoplasm, and nuclear injury could conceivably arise 

 through a disturbance in cytoplasmic metabolism. The dose 

 relations are not linear but those of an effect which varies with 

 the log of the dose. This suggests the existence of material, 

 essential to the passage of the cell through the critical phase, 

 which is inactivated at a rate which is some function of the 

 administered dose and repaired at a rate which depends on 



