4. The muscle membrane beneath both ma and mb plates was smooth, or had a few wide, shallow folds; me plates usually had wide, shallow subjunctional folds; numerous deep, narrow folds were characteristic of the md plate. The length of unmyelinated pre-terminal axon or the number of sole plate nuclei were not useful diagnostic features.5. Obvious foci of sarcomere convergence in the capsular sleeve region of dynamic bag1 and static bag2 fibres coincided with the location of motor plates. Additional contraction foci were observed in the extracapsular region of dynamic bag1 fibres where there was no motor innervation; contraction occurs principally in the outer half of these fibres. No foci of contraction or motor plates were observed in the extracapsular region of static bag2 fibres; contraction in these fibres is typically mid-polar. 6. In some poles local contraction of chain fibres centred on the location of me plates. In others, very localized contraction occurred distal to the sites of ma plates.

SUMMARY1. One hind limb of each of four cats was either chronically de-efferentated, or chronically de-afferentated, and perfused with buffered glutaraldehyde fixative. Up to three different muscle nerves were dissected from each limb, post-fixed in osmium tetroxide and embedded in Epon. Ultrathin transverse sections were mounted on Formvar-coated single-hole specimen grids so that all the fibres in each nerve could be examined individually by electron microscopy.2. Non-circularity was expressed as the ratio (0): axon cross-sectional area area of a circle with same perimeter'The degree of non-circularity of all the afferent axons, or all the efferent axons, in each muscle nerve was determined. The proportion of fibres cut through the paranodal region, or through the Schwann cell nucleus, was as expected for group I afferent and for a and y efferent fibres, but hardly any typical paranodal sections of group II or III afferent fibres were encountered which suggests that their paranodal arrangement differs from that of other groups. In a quantitative comparison of noncircularity in different functional groups, fibres cut through paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were rejected.3. All the y efferent fibres in one nerve were studied in a series of sections cut at 25 ,tm intervals. The degree of non-circularity was found to be relatively constant along the internode of most fibres when the values at paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were ignored.4. The value of 0 varied widely from 1-0 (circular) to 0-5 or less from fibre to fibre within every functional group. However, the mean value of qS was less for y axons (0.68) than for a axons (0.78), and less for group III axons (0.79) than for axons in groups I and II (both 0.84). When the results for all the nerves were aggregated, these differences were statistically very highly significant, as was the difference in qS between group I and a fibres. If values of qS < 0 5 were rejected, the difference between the between a and y fibres was still very highly significant. 5. The external perimeter (S) of a non-circular fibre differs from 2i times the diameter of a circle just enclosing the fibre (D). It is shown that S = 0 95 7TD for group I and II fibres, S = 0 90 nrD for a and group III fibres, and S = 0-85 iD for y fibres.6. The myelin period, or interperiod repeat distance, varied from 14 1 to 15-6 nm in different cats, implying radial shrinkage of the myelin sheath from 15 to 23% The myelin period in a particular cat was the same for several nerves, and the same for fibres in different functional groups.7. The possibility that repetitive firing of axons during fixation contributed to the varying degree of non-circularity is considered but rejected as unlikely.8. It is deduced that about 10 % radial shrinkage of the myelin sheath, but little or no osmotic shrinkage of the axon, occurred during fixation and rinsing. Further radial shrinkage of about 8 % in all components of the fibre probably occurred as a result of subsequen...

1. The distribution of cholinesterases in the carotid body of the cat has been investigated with the electron microscope to obtain a clearer picture of the localization of the enzyme. This is of relevance to the possible role of acetylcholine as a transmitter in the carotid body. 2. Tissues were fixed for short periods and incubated by the method of Karnovsky for the fine localization of cholinesterases. 3. The enzyme was found in two main sites. Butyrylcholinesterase was present on the type II cell membrane while acetylcholinesterase was found in groups of peripheral axons. No intracellular enzyme was found. 4. It is concluded that acetylcholine may be a transmitter in the carotid body and that it may be partly destroyed by enzymic hydrolysis. A possible explanation for some of the anomalous pharmacological findings is suggested.

1. The distribution of choline acetyltransferase in the carotid body of the cat has been investigated with the electron microscope to determine sites of enzymic activity. This is of relevance to the possible role of acetylcholine as a transmitter in the carotid body. 2. Tissues were fixed for short periods and incubated by the method of Kasa, Mann & Hebb, for the fine structural localization of choline acetyltransferase. 3. The enzyme was found in the cytoplasm of the type I cells and seemed to be associated with vesicles. No enzyme was found in the large nerve endings synapsing with the type I cell. 4. Whole carotid bodies were assayed for their choline acetyltransferase activity and significant amounts were found. 5. It is concluded that acetylcholine may be a transmitter in the carotid body and that it is synthesized in type I cells. A possible mode of initiation of chemoreceptor afferent impulses is suggested.

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