SECT. 6] OF THE EMBRYO 841 



when a cell seems uniformly stained, the dye may yet be in close 

 association with minute granules or globules, and may be registering 

 their internal pH. instead of that of the continuous phase in which 

 they are. Lucke centrifuged the eggs of Arbacia and Cumingia, so 

 that four layers appeared in them, a lipoidal one at the top, next 

 a clear empty homogeneous protoplasmic layer, then a layer of small 

 granules and at the bottom a layer of pigment granules. Such zoned 

 eggs placed in 1/40,000 neutral red or brilliant cresyl blue solutions, 

 took up the dye, but only as regards the granules. The clear layer 

 and the lipoidal layer remained quite unaffected, unless they were 

 exposed to the dye so long that they became granular, but that was 

 an irreversible change leading to death. The centrifuged eggs were 

 normal in that they would fertilise and develop into gastrulae, and 

 Lucke concluded from his experiments that the tint given by a pH 

 indicator in a cell was by no means a measure of the pH of its clear 

 protoplasm, but rather of its granules. This must, of course, be 

 admitted ; nevertheless, the average overall pH of a cell is a constant 

 of much interest, and worth investigating. 



The method of crushing the cell or cells, and so admitting the 

 indicator to contact with the cell-contents, is an old one, and a great 

 deal of work has been done with it. Its obvious disadvantage is that 

 it does not guard against the effects of cytolysis, but, on the con- 

 trary, actually involves them as part of its measurement^. Much work 

 has been done by this type of method on eggs, especially by the 

 Strasburg school. The earliest observation was that of Dernby, who 

 squashed the eggs of Strongylocentrotus lividus in indicator solutions, 

 and obtained a value of />H 6-5 for the interior. He did not, however, 

 follow up this line of work, and the technique was elaborated in 

 much detail by Vies. Vle^' apparatus differed little from what the 

 older microscopists used to call a " compressorium " ; the ^gg of an 

 echinoderm, for instance, was placed in it, surrounded by an in- 

 dicator solution, the ^gg was then compressed or crushed by turning 

 the screw, until the dye penetrated into the cytoplasm, upon which 

 the pressure was suddenly released, and the outside of the tgg washed 

 with sea water, or colourless solution. Vies found that the eggs of 

 Strongylocentrotus lividus, studied in this way, were yellow to brom 

 thymol blue and to bromcresol purple, but also yellow to methyl 



^ The same remarks apply to Tchakhotine's attempt to determine intracellular pW 

 by injuring echinoderm eggs in indicator solutions with ultra-violet light. 



