A DETAILED STUDY OF THE CHANGES OCCURRING IN THE PHYSIOLOGICAL DEGENERATION OF ACTINOSPHÆRIUM EICHORNI

For a clear understanding of the changes to be described in the series of Actinosphceria, the study of which forms the basis of this communication, it is necessary to give a description of a normal Actinosphcerium and a resum6 of Richard Hertwig 's work on the physiological degeneration of this animal. In life, a normal Actinosphcerium is a spherical cell with numerous radially projecting pseudopodia, regularly distributed over the surface, and each with a homogeneous axis fiber. The protoplasm is sharply divided into a coarsely vacuolated cortical layer, which contains the contractile vacuoles and through which the base~ of the axis fibers of the pseudopodia run; and an inner, more finely vacuolated protoplasm, the medulla, in which lie the nuclei and the food vacuoles. The nuclei are situated chiefly in the peripheral layer of the medulla, just beneath the cortex; but a few nuclei are scattered in the deeper parts of the medulla. The nun> bet of nuclei varies with the size of the animal. According to R. Hertwig, the nuclei, under normal conditions, vary from IO to 14 ~ in diameter, aim are surrounded by a distinct nuclear membrane, and contain nuclear sap and an achromatic linin framework~ on which lie the nucleolar substance and the chromatin. He regards the nucleolar substance as identical with the material of which the nucleoli Of the tissue cells of metazoa consists. According to Hertwig, in the simplest, but least common conditions, as