The resting spores of Ditylum are formed as a result of a peculiar kind of plasmolysis of the protoplast (Gross, 1939); they are small spherical bodies lying inside the old cell wall. Since one-half of the cell wall is only slipped over the other half it is likely that in the sea they might become separated and the resting spore proper would drop out of the shell. In a previous paper (Gross, 1937) it was stated that in old cultures a considerable number of resting spores could be found outside the parental shells, and when transferred into fresh culture medium they only expanded in volume. Since then more attention has been paid to such isolated resting spores and the formation of new cell walls has been frequently observed.