FACTORS IN EMBRYOGENESIS 



17 



evidently established at a very early phase in the development of the 

 zygote; that is, on the assumption that it v^as not already present in 

 the unfertilised egg. 



A single zygote, kept in darkness and as free as possible from 

 gradients or asymmetry of environmental factors, forms a single rhizoid 

 and develops mormally. Whitaker argues that a very labile and readily 



<s=, 



CSV- 



Fig. 3. Experiments on polarity in Fucus 



A, In a cluster of zygotes, the rhizoidal development is typically centripetal. B, 

 When a fertilised egg is placed in a gradient of pH, i.e. between two pipettes of 

 solution, the rhizoid grows out on the more acid side (A and B, after Whitaker). 

 C, Apparatus for determining the effect of various solutions on the segmentation of 

 Fucus; {see text pp. 18, 48) (after Olson and DuBuy). D, Segmentation in Fucus 



(after Rostafinski). 



altered polarity is either already established (perhaps due to conditions 

 during oogenesis) or that it is established after shedding by 'chance 

 environmental differentials of a minor sort.' Knapp (1931) states that 

 in Cystoseira — an alga of Sargassum affinity — the rhizoid emerges at 

 the point of entrance of the spermatozoid ; and this may also happen 

 in Fucus. The important point is that if polarity is thus established, it 

 is readily superseded by the effects of subsequent physical or chemical 

 gradients. 



The facts relating to the establishment of polarity may be summar- 

 ised as follows : Unilateral illumination by white fight causes rhizoids 



