Caryedon gonagra (F.) is a beetle pest in ground nut kernels and occurs in this country in stored nuts. The karyotype has been investigated in both sexes. The chromosome complement of 2n = i8 includes a heteromorphic sex-determining pair—a large metacentric X and a small spherical Y. Particular attention has been paid to the sex bivalent in the male since study of this character alone may prove sufficiently diagnostic in species separation in closely related members of the Bruchidae.

This discussion of the biochemical lesions produced in animal cells by ionizing radiation will to all intents and purposes be restricted to a consideration of the immediate response to doses of 1000 rads or less in two rat tissues only, namely the thymus gland and the regenerating liver. The experiments we have carried out with these two tissues have led us to the view that in these organs the ultimate damage to the cell stems from an initial upset of the nuclear thiol-disulphide equilibrium.

The study of chromosome aberrations produced by the irradiation in vivo of men dates from 1960, and nearly all the work has been done on cultures of blood in which lymphocytes respond to phytohæmagglutinin by synthesising DNA and then dividing (Moorhead et al. 1960). This technique can now be used on a moderately extensive scale and it is possible to trace the pattern of events after a single exposure or repeated exposures to radiation involving either the whole or part of the body, whether the exposure be for therapeutic purposes, or from the explosion of a nuclear weapon, or accidentally received, or incurred for occupational reasons. We can now detect the effect of radiation in producing an increased number of lymphocytes in the peripheral blood with chromosome aberrations with quite low doses of radiation, for example an occupational dose of about 15 to 20 rads. accumulated over a few years or the doses received during fluoroscopic procedures in diagnostic radiology. For the future we anticipate that the development of automated techniques of chromosomes counting and analysis will permit quantitative work down to such levels and perhaps even lower.

The experiments to be discussed here all involve the irradiation of cells in early interphase and the observation of these cells, or rather their chromosomes, at the first mitosis following radiation exposure. The irradiation of such G1 cells results in the formation of chromosome-type aberrations that are a consequence of breaks or exchanges involving the whole chromosome, both chromatids being affected in exactly the same way and at identical loci. These chromosome-type aberrations are of various kinds and a large proportion of them are very easy to detect cytologically. The four main types that are normally studied by cytologists, since they can be easily and objectively scored, are the terminal and interstitial deletions and the dicentric and ring aberrations (see text-fig. 1).

As the three previous papers in this Symposium have shown (Ord and Stocken 1968; Evans 1968; Court-Brown 1968) the exploration of radiation effects within the cell has its own intrinsic fascination quite apart from its relevance to an understanding of the phenomena which may be observed after irradiation of the complex organism, vertebrate or invertebrate. Whitmore (cf. Whitmore Gulyas and Botond 1965) has been one of the pioneer workers on a previously unsuspected phenomenon, the quantitatively substantial recovery of the ability to divide which occurs within the first hour or two after exposure to low LET radiation. In cultures of mammalian cells in vitro this kind of recovery was shown to have nothing to do with cell division, to occur at all stages of the cell cycle, and to be little, if at all, affected by a reduction in temperature which would be expected a priori to modify profoundly the rate of chemical reactions. Whitmore's technique for synchronising asynchronously dividing cell cultures by the selective suicide of DNA synthesising cells following the incorporation of radioactively labelled DNA precursors with high specific activity has a wide application (Whitmore and Gulyas 1966). Workers from his laboratory have proposed an interesting approach to the real meaning of “ability to divide” and “recovery” based on the idea that there is a certain probability of failure at each cell division and that this probability may be permanently increased by exposure to radiation (Till, McCulloch and Siminovitch 1964). Thus radiation damage may be expressed by failure of cell division not only at the first or second divisions after exposure but also at any subsequent division in the distant future.

I thank you very much indeed for the compliment you are paying me in inviting me to give the James Scott Lecture. It is always an honour to be asked to give a lecture which is, as the nursery-man would say, a “named variety”, and in addition I welcome this invitation from a Society of which I am proud to be an Honorary Fellow.

Your lecturer is invited to talk about “fundamental concepts of Natural Philosophy” and I confess that this set me quite a task. It is one thing to talk about some special branch of research in which one is interested; it is quite another to review in a broad way some general feature of science. It asks for a philosophical approach, and I am not only a poor philosopher but also in a country where philosophy seems to flourish on the native soil. I hope therefore that you will receive what I have to say indulgently.

It is the custom of our Society that the President of the day should at some period in his term of office address the Fellows. The manner in which he interprets this charge will, of course, vary according to individual preference, but for obvious reasons, he usually elects to give his personal interpretation of that area of science in which he is himself most deeply concerned.