FACTORS IN EMBRYOGENESIS 19 



exceed 0-4°C., (Lowrance 1937). Respiration and other processes will 

 proceed more rapidly on the warmer side; the pH gradient will also 

 tend to be established with the highest acidity on the warmer side. 



Centrifuging and Polarity. The centrifuge method developed about 

 the beginning of the century was taken up by many investigators. 

 The basic idea was that, if the specific organ-forming substances, 

 beheved to be present, could be displaced from their positions in the 

 cell, the positions in which new parts developed would be altered. The 

 centrifugal displacement of visible granules, as in gravity experiments, 

 was soon found to have no effect on polarity. Lillie (1909) concluded 

 from his experiments that polarity is a property of the ground substance, 

 i.e. the cytoplasm, while later Conklin (1931) considered that the effects 

 produced by high speed centrifuging in ascidian eggs were due to the 

 displacement of specific areas of cytoplasm. As Schechter (1934, 1935) 

 was able to change polarity in the red alga Griffithsia bornetiana by the 

 use of low speeds, he considers it improbable that displacement of 

 cytoplasmic areas took place. Cells of this alga, in which the dense 

 material had been centrifuged to the basal end, developed photo- 

 synthetic filaments and not rhizoids at that end. These filaments were 

 not so large as those developed at the apical end, nor was the develop- 

 ment of filaments at the apical end precluded. Rhizoids continued to 

 be formed in their normal positions. Schechter does not consider that 

 specific organ-forming substances were necessarily involved in the 

 developments observed, but rather that the formation of the new 

 filaments was due to a stimulus set up by an 'unusual concentration 

 of rather non-specific materials.' Centrifuged eggs of Cystoseira 

 became stratified and developed rhizoids at the centrifugal end 

 (Knapp, 1931). 



Schechter (1934) showed that Griffithsia, when placed in an electric 

 current, formed rhizoids towards the positive pole and also that the 

 chromatophores migrated to that pole. He then centrifuged the 

 chromatophores to the centrifugal end and found that filaments and 

 not rhizoids developed there. Whitaker (1937) has shown that when 

 Fucus eggs, embedded in sea-water agar, are centrifuged, they become 

 stratified; and when the eggs develop in normal sea water at pH 8-0, 

 with the stratification still present, the rhizoid develops at the centrifugal 

 pole. Fig. 4. Where the visible contents become redistributed before 

 the formation of rhizoids, the latter develop at random in relation to 

 the previous stratification. Ultra-centrifuging produced essentially the 

 same result. When ultra-centrifuged eggs were kept in sea water, 

 acidified to pH 6-0, the rhizoids formed centripetally. Fig. 4. (For a 

 tentative explanation of this result, see Whitaker, 1937). 



Beams (1937) and Whitaker (1937) have recorded that the eggs of 



