4. Making an arbitrary but reasonable assumption about the diffusion coefficient of K2SO4 in the inner unstirred region, the magnitudes of the inner unstirred layers were found to lie within the ranges 150-230 It, 120-200 /t and 100-170It under stirring conditions of 120, 300 and 500 rev/min, respectively.

SUMMARY1. Conductance changes in the acini of the cockroach salivary gland have been examined during nerve stimulation by means of two intracellular electrodes placed in the same acinus, the first electrode being used for recording membrane potential and the second for current injection.2. The transient hyperpolarization secretaryy potential) in the acinus evoked by nerve stimuli is accompanied by a rise in membrane conductance. The conductance, however, remains high for a longer period than that of the response.3. Applying the analysis of Trautwein & Dudel (1958) to the secretary potentials recorded in the acinus (assumed to behave electrically like a single cell) gives estimates of the 'transmitter equilibrium potential'. The values indicate that the neurotransmitter increases the membrane potassium conductance.4. The hyperpolarization of the acinus evoked by 10-6 M dopamine in the bathing fluid is also associated with an increase in membrane potassium conductance.

SUMMARY1. The morphology of physiologically identified spinocervical tract (SCT) neurones was studied using the intracellular injection of Procion dyes in anaesthetized and decerebrate cats.2. Extracellular recordings were made from SCT neurones at depths between 1000 and 2850 ,um from the cord surface but neurones were only stained at depths between 1100 and 2400 ,tm.3. The dendritic trees of stained SOT neurones were reconstructed in the transverse plane of the spinal cord. All SCT neurones had well developed dorsal dendrites but despite this it is not possible to consider the twenty-two SCT cells in our sample as constituting a morphologically homogeneous population. 4. There was no correlation between the form of the dendritic trees and the depth of SCT neurones in the dorsal horn as determined both from measurements from the dorsal grey-white border and the position of cells with respect to the border between Rexed's laminae II and III.5. Six types of SCT neurones were identified on the basis of the form of their dendritic trees as viewed in the transverse plane: (1) radially symmetrical, (2) semicircular, (3) large elliptical, (4) bilobed, (5) triangular, (6) small elliptical. Each of these types was found only in a certain region across the dorsal horn although any one region could contain more than one type.6. Spinocervical tract neurones with small elliptical dendritic trees always had receptive fields encompassing part of the hip or thigh and were unique in being located in the lateral portions of the horn.7. There was no correlation between the morphology of SCT neurones and their excitatory cutaneous inputs, receptive field size, axonal conduction velocity or depth in the dorsal horn.

SUMMARY1. Measurements of the diffusional permeability, Pd, of tritiated water in isolated frog skin bathed in sulphate Ringer have been made under different stirring conditions.2. The mean + S.E. values for Pd were found to be (6.5 + 0.3), (7.9 + 0-5), (9.7 + 0.7) and (11.1 + 0.8) x 10-5 cm sec-' at 120, 300, 500 and 1000 rev/min. It is considered that these results indicate the existence of 'unstirred layers' associated with frog skin.3. The hydraulic conductivity, LP, of the skin in sulphate Ringer was found to be (2.36 + 0.07) x 10-7 cm sec-1 atm-1 (± S.E. of estimate), and no marked increase in this value for Lp was found when the stirring rate was increased from 0 to 500 rev/min. 4. It is considered that these results show that previous comparisons of the relative magnitudes of LPRT/IV,, (where VF, is the partial molar volume of water) and Pd for frog skin have been in error because of the presence of 'unstirred layers'. 5. The bearing of our results and other evidence on the question of pores in cell membranes has been discussed.

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