THE cells of stratified squamous epithelium, whether normal or neoplastic, generally remain in contact with one another. The vascular stroma on which their nutrition depends lies in contact with the epithelium but the capillaries do not penetrate between the individual cells. Tumours composed of these cells often grow in solid rods which, when seen in histological sections cut in a plane at right angles to their axes, appear as circular areas surrounded by stroma. In tumours of this kind the centres of the larger areas are necrotic and are surrounded by intact tumour cells which appear as rings.It appeared that this particular structure might have an important bearing on the radiotherapy of such tumours, since there must exist a falling gradient in oxygen tension between the periphery and the centre of each tumour cord, and it is well known that cells which are anoxic at the time of irradition are generally much less damaged by a given dose of X-or y-radiation than those which are well oxygenated (Gray et al., 1953).The magnitude of the oxygen gradient depends on the consumption of oxygen throughout the cell mass, which is not accurately known, but simple calculations show that the degree of anoxia of the cells which are more centrally situated might be very significant. For example, complete anoxia would be expected at the centre of a cord only 150,t in radius if the respiratory quotient throughout the tumour mass were 5-3 ,1a. of oxygen/mg. dry weight/hour.-the mean Q02 of tissue slices from thirteen human tumours examined by Warburg (1930). We therefore undertook jointly a quantitative examination of many histological sections. We selected these from neoplasms which appeared to be growing in the manner described above. These tumours were mainly poorly differentiated squamous carcinomata.Tumours of this type arise in any site where stratified squamous epithelium develops, and some 40 per cent of the bronchial carcinomata so far examined have this structure. It is mainly on these carcinomata that our calculations have been based, though a similar pattern of necrosis with approximately the same dimensions has been observed in squamous carcinomata arising in other sites, and in some carcinomata from other epithelia, e.g., the stomach, the breast and the Fallopian tube. Histological materialFresh operation specimens of carcinoma of the bronchus were fixed in 5 per cent formalin. 4-7, sections were cut and stained with Erhlich's haematoxalin.

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Schmidt-Nielsen & Larimer (1958) made a comparative study of the oxyhaemoglobin dissociation curves of mammalian blood in relation to body size. Previous to this very few measurements seem ever to have been made of the relation between the oxygen tension and the degree of saturation of the unhaemolysed blood of mice or rats. This is strikingly illustrated by the fact that the figure for the oxygen tension at 50 % saturation quoted by Bartels & Opitz (1958) is taken from Prosser (1950), who in turn quotes from the observations made in 1912 by Douglas, Haldane & Haldane (1912) on blood taken from animals asphyxiated by drowning, which was almost certainly heavily loaded with lactic acid and not characteristic of the animal at rest or in moderate activity. Our object was to compare the oxygen-dissociation characteristics of the blood of the mouse, the rat and man, at identical PCO2 and pH.Two methods of determining the dissociation curves are described below, preceded by reference to the procedures employed for the micro-assay of oxygen and an account of the processes of taking and equilibrating the blood, which are common to both. METHODSTaking and equilibrating the blood The blood was drawn from the heart (in the case of human blood, from a vein in the arm) into a heparinized syringe. That from about a dozen animals was pooled and one drop of 1 % sodium fluoride added for every 3 ml. of blood. The blood was kept on ice until needed. Any adjustment of pH was carried out at this stage. Two millilitres ofthe agitated blood were drawn into a 30 ml. graduated syringe followed by successive amounts of nitrogen, oxygen and carbon dioxide appropriate to the required partial pressures. The PCO2 was approximately 40 mm Hg throughout the experiments (individual values noted in each table). The gas mixture occupied a volume of 28 ml. at room temperature. The needle of the syringe was closed with a short blind length of small-bore rubber tubing, and the syringe rotated for 15 min at 370 C in a water-bath. Care was taken to ensure that the plunger of the syringe was quite free to move inside the barrel, so that the gas was at atmospheric pressure. After 15 min the syringe was removed from the water-bath and the gas expelled. A fresh gas mixture of the same composition was introduced into the syringe and the blood re-equili- After a certain number of measurements had been made on blood drawn from the orbital sinus of anaesthetized mice, it was noted that this blood was invariably acid (pH 7-15) and therefore not comparable with human blood at pH -7-45. In all subsequent work blood was drawn from mice and lightly anaesthetized rats by cardiac puncture. In the case of the mouse blood, the pH of the blood as drawn was often lower than 7-45. In such cases a 2 ml. sample was equilibrated as described below, but for only one period of 10 min, with a mixture of air and carbon dioxide such that the PCO2 was 40 mm Hg. The pH of the blood was measured, and enough N-NaOH added from a microsyringe to the bulk of the blood (on the basis of...

The absolute measurement of neutron flux density by ionization methods is discussed.The relation Eυ = JυW ρ between the energy Eυ absorbed per unit volume of a solid medium and the ionization Jυ produced in a gas-filled cavity in the medium, the average energy W expended by the ionizing particles in producing a pair of ions in the gas, and ρ the stopping power of the solid relative to the gas for these particles, is critically examined from the point of view of the measurement of the rate of absorption of neutron energy at any point in a hydrogenous medium. The accuracy attainable is assessed, in so far as it depends on our knowledge of the quantities ρ and W.