CYTOPLASMIC DNA IN IRRADIATED NEURAL TUBE 83 



cause a rapid rise in the degenerate cell count, beginning at about 13 hours, 

 and since the dying cells are mitotic stages, this rise is in the hyperchromatic 

 type of granule. The smaller rise in the pyknotic stage shows that some cells 

 also break down before beginning division. 



So-called micronuclei are a well known and classic effect of radiation. 

 Although first seen by Koernicke ( 1905) in irradiated roots, it remained for 

 Alberti and Politzer ( 1924) to show in the corneal epithelium of salamander 

 lar\ae what becomes of a piece broken from a chromosome if it does not 

 rejoin. Lacking a centromere, it does not move to one of the poles in ana- 

 phase, but lags on the spindle and is usually not included in either daughter 

 nucleus at telophase. Remaining in the cytoplasm, it becomes spherical and 

 lies beside the chief nucleus. They named these Teilkerne or partial nuclei. 

 Se\eral isolated chromosome fragments may unite into a single larger micro- 

 nucleus ( Ohnuki and Makino, 1960). 



The great majoritv of so-called micronuclei are nonliving, spherical, deep 

 stainins; bodies about 2 /x in diameter. Those few acentric fragments that 

 contain both an adecjuatc amount of heterochromatin and a nucleolar or- 

 ganizer may continue to li\e, however, playing an active part in cell metabo- 

 lism and dividing synchronously with the main nucleus for several subse- 

 cjuent life cycles i LaC^our, 1953: McLeish, 1954'). The completeness of the 

 chromosome set is supposedly necessary for the normal functioning of the 

 cell; consequently, the daughter cell with the deficient chromatin, presum- 

 ablv the one in whose cytoplasm the micronucleus became enclosed, has been 

 assumed to be short-lived. However, Ohnuki and Makino (1960) pro\ed 

 sur\i\al throughout at least one mitotic cycle, and Hornsey 1956, 1960) 

 showed that their ma.ximum number appeared at the end of the first 

 mitotic cycle ( prolonged by irradiation ) and that their subsequent decrease 

 was exponential, depending only on dilution by further cell di\ ision. 



Chromosome breaks may occur trom exposure ot a cell to irradiation in 

 any stage of its life cycle. However, interphases in which the chromosomes 

 are widely dispersed show a special resistance to chromosome breakage as 

 compared to cells irradiated in the premitotic and mitotic stages when the 

 chromosomes are tightly condensed. Muller (1954) reviews the factors in- 

 \ol\ed. Lagging chromosome fragments are the chief change resulting from 

 irradiation in interphase, being few in early stages and becoming more abun- 

 dant as interphase progresses. Chromosomes irradiated in late prophase to 

 early telophase, even though eflfectively broken, give no e\idence of being 

 broken at the time, for when in the condensed condition they are held as if 

 by some enveloping material and can not fall apart into fragments. How- 

 ever, when the chromosomes recondense at the next mitosis, after an inter- 

 xenina: interphase has elapsed, more structural changes appear than would 

 ha\e followed irradiation in the interphase. Consequently, micronuclei are 



