SECT. 6] OF THE EMBRYO 843 



series of hollow wedges containing indicator solutions interposed 

 between the microscope and the light source. The accuracy of the 

 method was thus claimed to reach 0-02 pH, but this estimate has 

 seemed much too favourable to other workers on the same subject. 

 With the aid of this apparatus, Reiss found rhythmical variations 

 from pa 5'4 to 5-6 in the cell-interior during the first 250 minutes 

 after fertilisation, i.e. during the first two or three cleavages, and 

 he pointed out that these variations closely agreed with the rhythmical 

 elimination of carbon dioxide found by Vies (see p. 642), but as we 

 have already seen, there is reason for doubting these latter results. 

 Just as Vies' rhythmical results were not confirmed by Gray, so, as 

 will later appear, my wife and I were not able to find evidence in 

 favour of the existence of those described by Reiss. 



An alternative colorimetric method to that of microcompression 

 is, of course, that of micro-injection, whether of dissolved or solid 

 indicators. Vies himself, in his first paper on intracellular pH, said, 

 "The microdissection methods of Chambers in vv^hich one injects an 

 indicator directly into the cells, would be evidently the best, but 

 their difficulty hinders their general application". Actually the first 

 intimations of the value of this method were contained in the work of 

 Chambers, who in 1923 injected neutral red solutions into echino- 

 derm eggs, and observed that the tint taken up was always pink, 

 and never more than faintly orange. This direction of investigation 

 was followed up a little later by my wife and myself in a series of 

 papers (Needham & Needham). We injected the appropriate in- 

 dicators into the eggs of Strongylocentrotus lividus, and found that, 

 although bromthymol blue was always yellow in the cell, bromcresol 

 purple was always purple. This was in contradiction with the results 

 of the Strasburg workers with the microcompression method, as, of 

 course, was our final result of />H 6-5 for the intracellular hydrogen 

 ion concentration. On other eggs treated in the same way we 

 obtained the following values: 



Species /)H 



Sea-urchin (5<ro«^/ocen<ro<zw ZtwVzw) (unfertilised) ... ... 6-6 « 



„ (fertilised) 6-6 



Heart-urchin (Echinocardium cordatum) (unfertilised) ... 6-6 



Starfish (.4jfen<2j ff/a«a/w) (unfertilised) 6-6 



(fertilised) 6-6 



,, {Ophiura lacertosa) (unfertilised) ... 6-8 



Ascidian {Ascidia jnentula) (unfertilised) ... 6-6 



Polychaete worm {Sabellaria alveolata) (unfertilised) . . 6-6 



