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An investigation of isolated mitochondrial membranes, fixed with osmium tetroxide, potassium permanganate, or formaldehyde, shows that a great number of small particles are attached to their surfaces. Sections of specimens fixed in potassium permanganate or osmium tetroxide appear to confirm these findings. An attempt has been made to compare these particles with the ‘elementary particles’ described by Fernández-Morán and by Green, and also with particles revealed by negative staining of mitochondrial membranes, as reported by Stoeckenius. Specimens fixed with formaldehyde, whether isolated mitochondria or material fixed in bulk, show very poor contrast in electron micrographs.

Particles (regarded as molecules) of bovine serum albumin have been studied by electron microscopy both in the untreated state and after fixation with osmium tetroxide, formaldehyde, and potassium permanganate. All three fixatives, especially potassium permanganate, increased the density of the particles considerably.

The spermatid of the house-cricket, Acheta domesticus, has been studied by the use of vital dyes, histochemical tests, and electron microscopy. The acroblast consists of an outer and an inner part. The former, which contains lipids and a neutral mucopolysaccharide, is seen in the electron micrographs to consist of parallel lamellae. In section these appear to be arranged in the form of a horseshoe. The internal part of the acroblast gives feeble reactions for a neutral mucopolysaccharide and for a lipid. In electron micrographs it is seen to contain minute vesicles. The acrosome arises within the acroblast. It gives a strong reaction for neutral mucopolysaccharide. It appears to be devoid of internal structure. The Nebenkern formed by the fusion of all the mitochondria in the young spermatid, exhibits the ‘cristae’ characteristic of ordinary mitochondria. The reactions of this structure with vital dyes and acid fuchsine also parallel those of the mitochondria. Histochemical tests indicate the presence of a lipid. The periphery of the nucleus of the late spermatid shows a radial structure.

Values for the mean thickness of living bacilli in a number of different cultures of Lactobacillus bulgaricus were obtained from phase-change measurements. These were made with an interference microscope. The bacilli were mounted in media of several different refractive indices. The accuracy of these measurements is not limited by the wavelength of the light used, so that the mean thickness of bacteria can be measured more critically in this way than by an eyepiece micrometer scale. The values obtained for the mean thickness of the living bacilli were compared with the mean width of similar bacilli measured in electron-micrographs, after they had been prepared for electron-microscopy by drying them in air on formvar. The method used for preparing the bacteria for the electron-microscope is described in detail. It was found that osmium-fixed air-dried L. bulgaricus so prepared shrink linearly in the plane of the formvar film by 59%±5%, and similar unfixed bacilli by 51%±5%. Phase-change measurements through these air-dried bacilli indicated a similar degree of shrinkage, or a slightly greater shrinkage, in the direction at right angles to the formvar film.