In the smooth muscle of the rabbit uterus one type of myofilament can be found; this is about 50 A thick.

In the sarcoplasmic reticulum there are smooth walled vesicles and many vesicles with particles attached. It is suggested that these particles may correspond to the ribonucleoprotein-containing particles of Palade and Siekevitz. The diameter of the particles is 250 A. There are also unattached particles some of which may be glycogen.

The membranes of the muscle cells and the connective tissue lying between them are described.

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