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Slices of cerebral tissues from guinea-pigs or cats can be maintained in moist oxygen at the surface of salines in a condition which permits the measurement of resting membrane potentials (Li & McIlwain, 1957). This technique gives an opportunity of comparing the effects of substances on polarization and metabolic phenomena in the slices. The present investigation examines the effect of the following cations on the membrane potential: potassium, sodium, calcium, magnesium and ammonium. L-and D-glutamate were also examined in view of their effect on the tissue content of potassium salts. METHODSIn most experiments the cerebral cortex of the guinea-pig was used; occasionally experiments were done with rat, human, calf or sheep slices. Human specimens were obtained at operation for non-malignant conditions by Mr V. Logue and Mr P. Schurr, to whom we are grateful. Calf and sheep specimens were excised after the animals had been anaesthetized and used for other experiments by Dr F. Bell, to whom also we are grateful. Guineapigs and rats were exsanguinated and the brain excised as quickly as possible. Slices were cut tangentially to the surface of the parietal cortex, with a strip of razor blading, using a glass guide which gave slices 0 3-035 mm thick; these were transferred immediately from the blade into the incubating medium in the slice chamber (see below). The time elapsing between exsanguination and the beginning of incubation was 2j-3 min. Usually the superficial slice only was used, and it was pre-incubated for 40 min before recording.The normal bicarbonate glucose saline contained (mM): NaCl 124, KCI 5, KH2PO4 1-24, MgSO4 1-3, CaCl2 2-8, NaHCO3 26 and glucose 10, and was equilibrated with 5 % C02 in 02. Its pH was 7-2-7-4 at the beginning of experiments, and never exceeded 7-8 by the end. Other salines are described in terms of their difference from this normal saline. Apparatu8sThe apparatus was basically that of Li & McIlwain (1957), but differed in the following ways. The slices rested on a nylon grid mounted on a Perspex ring. The slice chamber and the water jacket which maintained the temperature at 37-38°C were made of a single piece of glass. In the assembled apparatus, a Tempunit pump supplied water to the outer jacket in a closed circuit from the thermostatically controlled bath. The 95 % 02 + 5 % CO2 gas mixture was passed through a flowmeter and a sintered-glass bubbler immersed in the

Cerebral slices cut from rat brain, either 2-3 mm or 0.27 mm thick, were used to study the effect of embedding and freezing. Paraffin wax sections 6 micrometer thick were mounted and stained with haematoxylin and eosin or Marsland et al.'s (1954) silver stain, and their areas were examined at each step. Embedding in paraffin wax of slices 2-3 mm thick, or in Epon of slices 0.27 mm thick, caused a diminution of their areas by 20-30%. Staining of paraffin wax sections did not alter their areas. Glycerol alone at 15% concentration had no effect on the areas, but at 30% concentration they were diminished by approximately 20%. Diminution of the areas of glycerol treated slices 0.27 mm thick also occurred when they were transferred to liquid N2 or to isopentane, but the areas increased after glycerol was replaced by Freon 12. It was concluded that embedding or freezing cerebral slices caused changes in their areas, but that staining of sections after they had been embedded, sectioned and mounted did not.

SUMMARY Cell bodies of cerebral neurons from rabbits were isolated by hand, transferred to a microscope slide in a ‘199’ medium, and the projected areas of their cytoplasm and nuclei were measured. In sixty‐four cells there was a strong correlation between the projected areas of the cytoplasm and the nuclei (r=0.66, P < 0001), and the ratio of the projected areas was 11.6. The medium was then replaced by the following fixatives: formalin (10% v/v), Bouin's, Carnoy's, Susas, glutaraldehyde (5% v/v), and osmium tetroxide (1% w/v). Cerebral slices were obtained from the grey and white matter of rabbit and rat, and were also measured before and after treatment with similar fixatives. Relative to the unfixed areas, glutaraldehyde and osmium tetroxide had no significant effect on the projected areas of isolated cells, Carnoy's fixative shrunk the areas of the cytoplasm by means of 16.26%, Bouin's by a mean of 49%, and Susas by a mean of 65%. The shrinkage of the cytoplasm and the nuclei was not significantly different from that of the nuclei for each of these three fixatives individually, but with formalin the mean shrinkage of the cytoplasm was 46% while the nuclei did not shrink significantly. Using the same fixatives the effect on the areas of the cerebral slides from rabbit and rat were as follows: glutaraldehyde and osmium tetroxide caused no change in area; Carnoy's, formalin and Bouin's fixative diminished the areas by a mean of 10–20%, and Susa's by a mean of 35%. It was concluded that a particular fixative often caused a different degree of shrinkage to the cytoplasm, nuclei and cerebral slice. In general, the lower the osmotic pressure of the fixative, the less shrinkage it induced.