L O C A L I Z A T I O N I N T H E D E V E L O P I N G F U C U S E G G
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2. Group Effects
As early as 1888, Rosenvinge reported the tendency of Fucus eggs to
germinate toward each other, the so-called positive group effect (Fig. 9 ) .
Whitaker found this tendency to be enhanced b y high cell concentrations
(Whitaker, 1931b) and b y low p H (Whitaker, 1937). In fact, isolated
pairs of eggs in the relatively high p H range of about 7.9 to 8.7 actually
show a strong negative group effect (Fig. 1 0 ) . Moreover, individual eggs
grow toward and away from nearby diffusion barriers to a degree and
with a dependence upon p H quite similar to their mutual polarization
(Whitaker and Lowrance, 1937). F r o m this and other evidence (which
follows) it can be inferred that these interactions are mediated b y diffusion gradients of (at least) two substances—a stimulator of rhizoid
initiation which I shall call "rhizin" and an inhibitor which I shall call
"antirhizin."
Whitaker also found that individual eggs exposed to p H gradients
germinate toward the acidic pole (Whitaker, 1938) and that the eggs proFIG. 8. Eight-hour-old Fucus egg showing typical polarized plasmolysis. It would
have later formed a rhizoid toward the right. (From Reed and Whitaker, 1941.)