PROTOZOA AND INVERTEBRATE EGGS 80") 



doses. 'I'he other organisms were from various phyla whose eggs have 

 determinate cleavage, are immature when shed, and cleave spirally. 

 They showed little difference in sensitivity between the egg and sperm 

 and showed irregular cleavage at low doses. Henshaw et al. (1933) 

 report marked differences in sensitivity to cleavage delay by X rays 

 between Chaetoptenis, Nereis, Cumingia, and Arhacia eggs. 



Localization. A considerable amount of evidence in regard to the part 

 of the cell which is responsible for radiation-induced cleavage delay has 

 been accumulated. It rather strongly suggests a nuclear effect for the 

 sea urchin but is not so clear for the protozoa. Of course, the delay need 

 not be due to the same causes in such different types of cells, although 

 such a unifying hypothesis would be attractive. Thus the details of the 

 mitotic process in the ciliate protozoa are quite different from those in the 

 sea urchin. Moreover, in the eggs, there is no growth in size between 

 cell divisions, whereas in ciliates growth occurs, and is, perhaps, essential 

 for later divisions. 



In the sea urchin, it has been repeatedly shown that irradiation of either 

 the sperm or the unfertilized egg can bring about cleavage delay. Among 

 reports on this subject may be mentioned those of Henshaw and Francis 

 (193()) and Henshaw (1940a, b) on X-irradiated Arhacia; Marshak 

 (19-l:9b) and Blum, Robinson, and Loos (1950) on Arhacia exposed to 

 ultraviolet; Giese (1939b, c; 1946b) on a variety of marine invertebrates 

 exposed to ultraviolet. The two gametes obviously contribute quite dif- 

 ferently to the zygote. Thus the sperm contributes the male pronucleus 

 and the centrosome which functions in the first cleavage (Henshaw and 

 Francis, 1936). The egg contributes the female pronucleus and the bulk 

 of the c^ytoplasm. As Henshaw and Francis (1936) point out, the delay 

 produced by irradiation of the unfertilized egg indicates that injury to the 

 centrosome is not involved since this gamete does not contribute a func- 

 tional centrosome. The one portion of the zygote to which both gametes 

 are known to contribute is the nucleus. Therefore, the simplest conclu- 

 sion would be that the effect is on this structure. This conclusion is not 

 absolutely demonstrated by such evidence since it is possible that the 

 sperm contributes cytoplasmic elements which, though small in bulk, are 

 important in division. Nonetheless, the very fact that irradiation of 

 either gamete produces delay certainly suggests a nuclear effect. This 

 conclusion is not affected by the difference in sensiti^'ity between egg and 

 sperm, for the nuclei in the two gametes are in quite different physical 

 states and are subject to different amounts of shielding in the case of 

 ultraviolet. 



Supporting evidence for a nuclear site of the injury is furnished by 

 experiments with eggs fragmented by centrifugation. Henshaw (1938) 

 has shown that X irradiation of either the whole Arhacia egg or the 

 nucleated half results in cleavage delay but X irradiation has no effect on 



