The Behaviour of Meiotic Chromosomes After X-Irradiation

S a result of the work of numerous cytologists using a wide raiige A of both animal and plant material.the general outlines of chromosome behaviour at meiosis have been clearly demonstrated.There is still disagreement on several important points notably on the nature of the chiasina and its relations with crossing-over and on the relations between mitosis and meiosis.All the hypotheses put forward to explain these disputable points have one thing in coninion viz.they are all bused on inferences drawn from observations on the behaviour of normal chromosonies during somatic and reduction divisions.During the last few years, however, it has been conclusively proved that chromosomes can be altered morphologically and Functionally by treatment of the organism with X-rays and the y-rays of radium.The study of such abnorm:il cliroinosome types has lead to important advances in our knowledge of chromosome behaviour in a number of organisms particularly Drosophila and Zea.This work has been mainly concerned with the behaviour of chromosomes long after treatment, indeed having been largely performed on the offspring of treated organisms.It is known, however, that the direct effect of X-radiation differs with the type of cell division under consideration.In somatic divisions the chief effect is the production of major structural changes.In gametic mother-cells undergoing reduction division the result of similar treatment is of a more violent nature often resulting in the complete disruption of a large part of the chromosome complement.The reason for this difference must be sought in the different influences which are affecting the chrornosoines in the two types of division.Therefore a careful comparative study of the hehaviour of the cliromosomes immediately after irradiation must throw some light on the nature of the differences between the two divisions and the reason for the typical response of the chromosomes to these changing influences.