ABSTRACT
By means of a mercury manometer connected to a micropipette, the internal hydrostatic pressure of the unfertilized Fundulus egg was measured. Upon activation of the egg by micropuncture when the platelets in the cortex of the egg proper disappeared, there was a progressively increasing internal pressure which finally stabilized at 150 mm. Hg. The pressure was accompanied by the formation of a perivitelline space which contained a clear, colloidal material. The measured internal hydrostatic pressure of the egg was diminished by any means which decreased the perivitelline space. It was concluded that the perivitelline substance is the chief factor contributing to the pressure. The changes in volume of the egg proper are inversely proportional to the size of the perivitelline space, indicating that the surface of the egg proper is permeable to water.
The extensive work on the action of acids and various salt solutions on the Fundulus embryo is not discussed in our papers which are concerned solely with water permeability. A great difference also exists between the stages of development. In our work only freshly stripped eggs were used, and then only in the first few hours.
We are much indebted to Dr J. P. Trinkaus (Department of Zoology, Yale University) for his generous help in this operation.
