Theodor Schwann (1810-1882) founded the Cell Theory, Schwann argued then that all living cells and their nuclei are surrounded by a membrane, and that this membrane is prior in importance to its content.  The content  of the cellular cavity, i.e., of the interspace between the nuclear and cellular membranes was to Schwann typically a homogenous, transparent liquid.  In this view, the cell membrane possessed "metabolic power" by which  the membrane chemically altered the fluid substances adjacent to it - the Zellenkeinstoff outside and the cell content within.




Reference:
1. A Revolution in the Physiology of the Living Cell. Gilbert N. Ling.  Kreiger Publishing Company. 1992.