RECONSTITUTIONAL PATTERNS IN EXPERIMENT 425 



Polarity of Fucus eggs may also be determined by centrifugal force. 

 After a certain intensity and period of centrifuging eggs suspended in firm 

 agar-sea water to prevent possible orientation, 93-99 per cent of eggs with 

 persisting stratification develop rhizoids at or near the centrifugal pole, 

 but redistribution of egg substances abolishes this effect (Whitaker, 

 19376). According to Beams (1937), ultra-centrifuging does not affect 

 polarity of eggs of another species of Fucus. Whitaker (19386) finds, how- 

 ever, that at pH 7.9-8.1 the H-ion concentration of sea water, all ultra- 

 centrifuged eggs form rhizoids on the centrifugal halves, while at pH 

 5.8-6.0, well on the acid side, 90 per cent form rhizoids on centripetal 

 halves. He makes the further suggestion that auxin may be adsorbed on 

 heavy substances and attain inhibitory concentrations at the centrifugal 

 pole in acid sea water, or its transport may be affected by amphoteric 

 substances, with reversal when the isoelectric point is passed. Eggs ex- 

 posed to a slight temperature gradient develop rhizoids on the warmer 

 side (Lowrance, 1937a, h). When recently fertilized eggs are drawn into 

 a capillary pipette with lumen small enough to elongate the egg, the 

 pectin or cellulose wall "sets" and the egg retains ovoid form after extru- 

 sion: in darkness 96 per cent (of 114 eggs) form rhizoids at or near one 

 end of the longitudinal axis. At pH 6, however, rhizoids tend to form on 

 the surface in contact, where diffusion is less rapid, and this differential 

 supersedes the effect of shape (Whitaker, 1938c, 1940).^^ Whatever the 

 physiological factors concerned, it is evident that, even though a polarity 

 may be originally present in the egg, the polar pattern can be reconsti- 

 tuted by various external differentials, entirely without section or other 

 injury to the egg. 



In many animal eggs cytoplasmic differentiation is apparently so far 

 advanced at the beginning of embryonic development that axiate pattern 

 appears highly stable, but alteration is still possible in some eggs. When 

 polarity is obliterated by differential inhibition in the blastula of the 

 hydromedusa Phialidium (pp. 168-69), the blastula becomes solid and 

 spherical, and after return to water rolls about on the bottom of the con- 

 tainer for perhaps several days, ciliary activity being no longer co-ordi- 



3' For determination of dorsiventrality in an echinoderm egg in a similar way and the 

 interpretation suggested see p. 427. More recent papers on experimental determination of 

 polarity in Fucus and the related form, Pelvetia, are as follows: Whitaker, 1940, "The effects of 

 ultra-centrifuging and of pH on the development of Fucus eggs," Jour. Cell. Comp. Physiol., 

 15; Whitaker and Lowrance, 1940, "The effect of alkalinity upon mutual influences deter- 

 mining the developmental axis in Fucus eggs," Biol. Bull., 78; Lowrance and Whitaker, 1940, 

 "Determination of polarity in Pelvetia eggs by centrifuging," Gro2vth, 4. 



