This paper reports the common occurrence of creases or sharp ridges on the membranes of the mitochondrial cristae. In section these appear as sharp angulations recurring at more or less regular intervals along the profile of the crista. In instances where such angulations occur alternately on one membrane and then on the other, the crista has a zig-zag course. Where they occur at the end of a crista its profile has a square tip. An exaggerated expression of this tendency for angulation of the internal mitochondrial membranes is found in certain bat muscles where some of the cristae take the form of parallel bundles of prismatic tubules which are triangular in cross-section. Angular configurations of the cristae have been observed after various methods of specimen preparation, in a variety of cell types, and in a wide range of animal species. They are believed to be a normal variation of the basic structural organization of the mitochondrion and to be the morphological expression of a property which the internal mitochondrial membranes do not share with other membranes in the cell.

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