Amoeba proteus: Studying the Contractile Vacuole by Micropuncture.

Direct measurements of the freezing point depression of the protoplasm and of the fluid from the contractile vacuole of fresh-water amoebae showed that the fluid in the vacuole is distinctly hypoosmotic to the protoplasm. Fourteen samples of protoplasm from amoebae, placed in a medium with a milliosmolality of 7, had an average osmolality of 101 milliosmoles with a range of 73 to 116. Eleven samples of vacuolar fluid had an average osmolality of 32 milliosmoles, with a range of 24 to 38. It is suggested that the fluid may be isoosmotic to the protoplasm when secreted and that salt is subsequently reabsorbed, leaving the vacuolar fluid hypoosmotic to the protoplasm.