FACTORS IN EMBRYOGENESIS 21 



Fucus can be subjected to centrifugal forces of 150,000 times gravity 

 for several minutes without being killed or even suffering ill effects. 



In contrast to these results of Whitaker, when Beams (1937) strongly 

 centrifuged (150,000 x g) the eggs of Fucus serratus for various 

 intervals of time, the eggs showed marked stratification but this did not 

 appear to bear any particular relationship to the position in which the 

 rhizoid subsequently developed. He therefore concluded that the 

 stratification of the egg contents had little or no effect on its polarity. 

 (For a general review of the effect of centrifugation on plant cells, see 

 Beams and King, 1939). 



Unilateral ultra-violet irradiation of Fucus eggs resulted in rhizoid 

 development on the side away from the source of light. A sufficiently 

 strong dosage inhibits rhizoid formation without causing cytolysis. 

 Such dosages may act by denaturing proteins, by changing the pH, or 

 by inactivating the auxin (Went and Thimann, 1937). 



From the data set out above it appears that in plant zygotes or spores 

 the polarity may be determined to a considerable extent by external 

 factors, Stahl (1885) had already observed that differential exposure 

 to light determined polarity in the spores of Equisetum, while the 

 extensive researches of Whitaker and others have shown that the 

 polarity of the fertilised eggs of seaweeds can be modified by factors of 

 very different kinds. Among these, gradients of specific substances are 

 important. 



Polarity in Animals. In the zoological field Needham (1942) has 

 referred to the 'difficult question of polarity, perhaps the central puzzle 

 of embryonic development' (p. 656). With a wealth of illustration he 

 shows that the cell protoplasm must be envisaged as possessing 

 organisation : its constituent molecules, of many kinds, are arranged 

 in a particular way, or have a particular orientation, so that the whole 

 has a definite structure which, in morphogenesis, is manifested as 

 polarity. The polarity of the animal egg is in a high degree fixed, and 

 unlike some plant zygotes, e.g. Fucus, almost impossible to alter. 



