CELL DIVISION, MORPHOLOGY, VIABILITY 



771 



egg had been X-rayed, at short time intervals after insemination to com- 

 pare their progress with that of untreated controls. He found no retarda- 

 tion from entrance of the sperm head through fusion of the pronuclei; 

 considerable retardation from crescent formation (early prophase) 

 through late prophase, and slight retardation at metaphase, anaphase, 

 and telophase (Fig. 11-3). 



The relative radiosensitivities of the different stages of the sea urchin 

 egg from insemination to first cleavage were tested by Yamashita, Mori, 

 and Miwa (1939) and Henshaw and Cohen (1940). Their results are in 



MITOTIC TIME SCHEDULE 



NS + NE 123 

 NS + IE 



123 



IS + NE 



123 



5 678 



6 7 8 





6 7 8 



a 



IS +NE' L I_2_3 



Fig. 11-3. Diagram to show the effects of X radiation on the duration of the different 

 stages of the first cleavage division of Arbacia. At top is a semidiagrammatic repre- 

 sentation of the mitotic time schedule of Arbacia {patterned after Fry, 1936). 1, 

 Entrance of sperm head into egg; 2, elaboration of sperm aster; 3, fusion of pronuclei; 

 4, crescent formation and disappearance; 5, prophase; 6, metaphase; 7, anaphase; 8, 

 telophase. N, normal ; I, Irradiated ; S, sperm ; E, egg (after Henshaw, 1940b). 



close agreement. The former investigators, using Pseudocentrotus, found 

 that maximum delay of the first cleavage division results when irradiation 

 (X, 7, or (3 rays) takes place during the period extending from fusion of 

 the pronuclei into early prophase. Treatment during approach of the 

 sperm to the egg nucleus was somewhat less effective, while treatment 

 during late prophase, metaphase, anaphase, and telophase caused little 

 delay. Henshaw and Cohen (1940) X-rayed similar samples of Arbacia 

 zygotes at 5-minute intervals during the first 45 minutes after insemina- 

 tion. Using the Arbacia mitotic time schedule of Fry (1936) (see Fig. 

 11-3) to determine the stage at which each sample of zygotes was irradi- 

 ated, these investigators concluded that sensitivity as measured by 

 cleavage delay was greatest in zygotes treated during fusion of pronuclei. 



