

B 



, ~\ 



H 



\ 



^%2Siii.i*** 



v....^ 







a^ F 



'%>^,,._v.'-'" / 



Fig. 4. A-D^, The effect of centrifuging Fucus zygotes 



A, A fertilised egg 23 min after being centrifuged for 20 min at 3,000 x g; the 

 centrifugal end is below; the chloroplasts have become aggregated in the dark 

 centripetal band. B, The same egg 1 5 min later; some dispersion of the dark band 

 is taking place. C, Another similar egg, 3 hr 10 min later; the nucleus has emerged 

 from the dark stratum. D, Another egg from same sample 13 hr 25 min later; the 

 rhizoid has emerged at the centrifugal end. A^ and D^ are normal non-centrifuged 

 eggs, corresponding to A and D respectively. All these eggs were reared in the 



dark at 15°C and at pH 7-9-80. 



E-G^, The effect of ultra-centrifuging Fucus zygotes. 



E, Typical stratified egg soon after being centrifuged for 5 min in normal sea water 

 at 150,000 X g; the centripetal layer is the oil cap; the dense layer includes the 

 plastids and nucleus. F, Similar egg after development in the dark in sea water 

 at pH 8-0; the rhizoid has developed at the centrifugal pole. G, Later stage of 

 same. FS Egg similar to that shown in E after developing in sea water acidified 

 to pH 60; the rhizoid has developed at the centripetal pole. GS Later stage of 



same. 



H-L^, Effect of shape on the determination of the axis in 



Fucus zygote 



H. Normal spherical egg soon after fertilisation. W, Similar egg after rhizoid has 

 developed. K, An egg which has been elongated soon after fertilisation. K\ The 

 rhizoidal development in sea water at pH 8-0. L, An egg which has been elongated 

 soon after fertilisation. L\ The rhizoidal development at the end of the short axis 

 in acidified sea water at pH 6-0; the rhizoid formed towards the bottom of the 



culture dish (all after Whitaker). 



