1. Gradients of hydrogen ion concentration across Fucus eggs growing in sea water determine the developmental polarity of the embryo.

2. Gradients may determine polarity even if removed before the morphological response begins.

3. The rhizoid forms on the acid side of the egg unless this is too acid, in which case it develops on the basic side of the egg.

4. Since gradients of hydrogen ion concentration in sea water produce gradients of CO2 tension, as a result of chemical action on the carbonate buffer system, it is not proven whether the physiological effects are due to the hydrogen ions, or to the CO2 which they produce in the medium.

5. The developmental response of the eggs to gradients of hydrogen ion (or CO2) concentration provides an adequate but not an exclusive explanation of the group effect in Fucus.

6. Hydrogen ions may exert their effect by activating growth substance. Hydrogen ions or CO2 probably also affect the underlying rhizoid forming processes in other ways as well.

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