The history of human muscular dystrophy with reference to clinical, histological and biochemical studies is reviewed. The value of the recent discovery of an experimental mouse with hereditary muscular dystrophy with clinical, histological, and biochemical similarities to human muscular dystrophy is discussed. The histology of normal muscle, with special reference to ultrastructure, is also reviewed. For this investigation mouse muscle samples for the electron microscope were fixed in Palades osmic acid solution, embedded in methacrylate or epon and stained with either lead hydroxide or phosphotungstic acid. A method for obtaining one day old muscle samples, while keeping the animals alive until the dystrophic symptoms are noted clinically, is outlined. Light microscope sections were obtained from electron microscope blocks and examined after staining with toluidine blue. Experimental results by light and electron microscope observations showed there to be no histological differences between one day old dystrophic and normal muscle. However one day old muscle showed histological differences compared to older muscle. The chief differences were the smaller size of fibers, random distribution of mitochondria and enlarged nuclei, and the presence of abundant interfibrillar sarcoplasm with a conspicuous granular components. The regular repeating pattern of the endoplasmic reticulum of adult muscle fibers was hot seen. Some "atypical" fibers showed similarities to altered fibers seen in older dystrophic mice. The mitochondria were swollen and vacuolated with few cristae and pale matrix. Endoplasmic reticular components were vacuolated and adjacent myofibrils were disorganized. Atrophy of fibers was first noted at 14 days of age and were conspicuous at 63 days. By 63 days alterations were also noted in mitochondria and endoplasmic reticulum and these changes became more prominent with the progress of the disease. Atrophy of the myofibrils was evident and the Z-bands were often irregular and out of register. Connective tissue also increased greatly. Several miscellaneous structures are also discussed. The histological findings are compared to those found by other workers. Several suggestions put forward by others as to the possible cause of the disease are summarized. On the basis of morphological findings it is suggested that ribonucleo-protein synthesis in the nuclei of dystrophic muscle fibers is increased but that there may be some intermediate stage at fault which prevents the conversion of amino acids to myofilaments. / Medicine, Faculty of / Graduate