On the similar orientation of fibrillar structures and rows of esterolytic invaginations in plasmodia of the slime mold, Physarum confertum Macbr.

The localization of thiolacetic acid esterase activity in plasmodia of the slime mold, Physarum confertum Macbr., permitted the visualization of a system of esterolytic invaginations of the plasma membrane which appeared closely related to the arrangement of fibrillar structures. Fibrils running perpendicularly to the long axis of tubes and channels of the conducting system were found to be attached to relatively large lateral infoldings, while rows of smaller invaginations seemed to follow the course taken by the fibrils. In living and fixed specimens, constrictions in the diameter of tubes and channels were observed at the level of fibrils and linearly arranged rows of infoldings. These findings suggest that esterase‐positive infoldings may result from the contraction of fibrillar structures which are attached to the plasma membrane at multiple points along their length.