Rods, double cones which comprise a principal and an accessory cone, and two types of single cones occur in the chick retina. The rod is narrow and contains a paraboloid, a basal nucleus and a small synaptic body with long synaptic lamellae. The principal cone has a pale-staining oil droplet and is aligned against a n accessory cone, which has a paraboloid, no oil droplet, but a small granular vesicle instead. The synaptic body of the principal cone is large and partly surrounds that of its associated accessory cone. The synaptic body of the accessory cone has a long process extending into the outer plexiform layer. Single cone type I has a dark oil droplet and the mitochondria in the ellipsoid are dense with cristae. Single cone type I1 has a lighterstaining oil droplet and fewer cristae. The synaptic bodies of both types of single cone lie vitreal to those of rods and double cones. Fibres connect the synaptic body to the nuclear region in single cones and accessory cones, but not in rods and principal cones. Another type of synaptic body, which is not that of a receptor, occurs in the outer plexiform layer. Groups of small vesicles, like those in the synaptic body, occur near the base of the inner segments; synaptic vesicles may originate here. Muller cells separate each receptor, except the members of a double cone, at the outer limiting membrane and may position the receptors in relation to the pigment epithelium to allow a radial orientation of the inner and outer segments.The visual receptors in many vertebrate retinas show a remarkable diversity of form (Walls, '42). In the chick retina, rods, double cones each comprising a principal and an accessory cone, and single cones with either a red or yellow oil droplet were recognised long ago (Schultze, 1867). An electron microscope study of these receptors in the chick has not been reported, though in some general descriptions of the ultrastructure of vertebrate retinal cells (Villegas, '60; Pedler, '65), the chicken retina was amongst the material examined. The present report is a description of the fine structure of receptor types in the chick retina. The description is based on radial sections of the retina in which a single receptor could be traced along its entire length from outer segment to synaptic body. It has thus been possible to describe the synaptic body of each type of receptor.The general ultrastructural features of vertebrate retinal receptors are well known and have been reviewed by Cohen ('63b). Ultrastructural studies of avian retinal receptors include those on the pigeon (Cohen, '63a) and weaver-finch (Yasuzumi et al., '58). There have been few classifications of the types of retinal receptors in one species of vertebrate, such as that carried out by Nilsson ('64a) on the leopard frog. Our aim is to present such an account for the chick. It forms a basis for future descriptions of the connexions between receptors and cells in the bipolar layer and for an understanding of the role of different receptors in visual function.MATERIAL AND...

SHORT COMMIUNICATIONS 27c pronounced change in the ratio of the trans to the cw forms, with the concentration of the former always being several times greater than that of the latter. There was little evidence of double-bond migration under normal conditions. The conjugated linoleic acid and linolenic acid have retention volumes greater than those ofthe normal compounds and could be separated by gas-liquid chromatography (Daniels & Richmond, 1960). No conjugated linoleic acid and linolenic acid could be detected either during control periods or during periods in which oleic acid was infused. Conjugates accounted for 5-10% of the polyenoic acids during infusion of small amounts of nonconjugated linoleic acid and linolenic acid and 15-25% during infusion of larger amounts. The conjugated polyenoic acids are hydrogenated with difficulty (Garton, 1959) and would be expected to accumulate to a much greater extent than the non-conjugated acids. The above results offer no support for the hypothesis that the process of hydrogenation involves appreciable bond migration and accumulation of conjugated acids (Shorland et al. 1957; Kepler et al. 1965), but they are consistent with the hypothesis that elaidic acid, and not oleic acid, is an intermediate in the hydrogenation of the polyenoic acids in the rumen. We are grateful to Miss G. H. McG. Breckenridge for her assistance with the analyses.

The proliferation and secretory activity of the uterine endometrium in the marsupial T. vulpecula is examined at the cellular and subcellular levels throughout the 26-day oestrous cycle. The observations described are correlated with measured concentrations of progesterone in the peripheral blood plasma. Evidence cited indicates that there are no significant functional differences in the uterine endometrial secretory activity during the 17.5-day gestation period in pregnant females, compared with those in a normal oestrous cycle. Progesterone assays carried out on blood plasma taken from 20 staged animals throughout the oestrous cycle, five of which were at known stages of gestation, also supports the view that pregnancy does not significantly alter the physiological pattern of the reproductive cycle in this marsupial.

The cell envelope of Halobacterium halobium was seen in thin sections of permanganate-fixed cells to consist of one membrane. This membrane appeared mostly as a unit membrane but in a few preparations it resembled a 5-layered compound membrane. The cell envelope of Halobacterium salinarium at high resolution was always seen as a 5-1ayered structure different in appearance from the apparent compound membrane of H. halobium. The "envelopes" which were isolated in 12.5 per cent NaCI from each organism were indistinguishable from each other in the electron microscope and comprised, in each case, a single unit membrane with an over-all thickness of about 110 A. Some chemical analyses were made of isolated membranes after freeing them from salt by precipitating and washing with trichloroacetic acid. Such precipitated membranes consisted predominantly of protein, with little carbohydrate and no peptido-aminopolysaccharide (mucopeptide). Sectioned whole cells of H. halobium contained intracellular electron-opaque structures of unknown function.