November 19, ]9]5] 



SCIENCE 



735 



the sp. gr. reading a coefficient is obtained 

 which is of use in reducing sp. gr. readings to 

 salt content. For tlie range of concentration 

 likely to be seen at Wood's Hole, i. e., when 

 sp. gr. 15° C./4°C. = 1.0210 to 1.0245 corre- 

 sponding to a total salt content of 2.84 per cent, 

 to 3.29 per cent., the salt content is obtained 

 with a probable error less than 2 in the second 

 decimal place by multiplying the sp. gr. read- 

 ing by the factor ISJf.B. 



EoDNEY H. True 

 U. S. Department or Agriculture 



ON" CELL PENETRATION" BY ACIDS'" 



Preliminary Note 

 1. The water-soluble blue pigment^ in the 

 cells of a nudibranch, Chromodoris zehra 

 Heilprin, is a sufficiently delicate indicator to 

 justify its use for the study of cell penetration 

 by acids. Water extracts of the animal, con- 

 taining this pigment and other cell materials 

 expressed by grinding, change from a deep blue 

 color with reddish-purple fluorescence to a 

 delicate pink hue at a hydrogen ion concen- 

 tration of pH='5.62; the acidity of the body 

 fluids of Chromodoris averages PH=7.4 (27°). 

 The indicator promptly flocculates, in the form 

 of a greenish-blue precipitate, leaving a blue 

 solution, at ph^^Y.6. Within the epidermal 

 cells the pigment is also turned green, so that 

 it may be used to measure the penetration of 

 alkalies ; it gives results concordant with those 

 obtained with a great variety of tissues by the 

 neutral red method (Harvey*), and with 

 neutral red-stained Chromodoris cells lacking 

 the blue pigment. 



1 Contributions from the Bermuda Biological 

 Station for Research, No. 39. 



2 Crozier, W. J., 1914, Journ-al of Physiology, 

 Vol. 47, p. 491. 



3 This point changes some"what with the age of 

 the extract, in the ease of alcohol (95 per cent.) 

 and other permanent solutions of the pigment. 

 The Ph values given were obtained by titration 

 with phosphate and acetate mixtures, checked by 

 gas chain measurements on alcohol and formalin 

 solutions of the pigment. 



4 Harvey, E. N., 1914, Papers from the Tortu- 

 gas Lab., Vol. VI., p. 133. 



The pigment occurs in two forms : as gran- 

 ules scattered through the superficial and 

 deeper tissues, and dissolved in clear globular 

 bodies located within the cells of the outer 

 epithelium, especially along the edges of the 

 mantle and foot. It is totally insoluble in 

 anhydrous acetone, ether, chloroform, xylol 

 and oils. The globules containing it do not 

 stain with fat dyes. I conclude that the pig- 

 ment is held naturally in water solution. 



2. Direct measurements of the speed with 

 which acids penetrate protoplasm were first 

 given by Harvey,^ who determined the 

 time required for the testis of Stichopus 

 ananus to change in color when immersed in 

 0.01 TV solutions of a number of acids. I have 

 used pieces of the lateral mantle edge of 

 Chromodoris in a similar way, precautions 

 being taken to insure comparative uniformity 

 of the pieces in the different tests, and find 

 that at this concentration (0.01 N) the acids 

 employed when arranged in the order of in- 

 creasing penetration-time form the series 

 shown in Table I. Comparison of this list with 



table I 

 Penetration of Acids from 0.01 N Solutions 



5 Harvey, E. N., 1914, Science, N. S., Vol. 

 XXXIX., p. 947. 



Only those acids which I have studied have 

 been taken from Harvey's table, which includes 

 a number of others. 



" The differences in penetration-time for Nos. 

 5-8 are slight at this concentration, but their sep- 

 aration is justified on the basis of the dilution 

 curves. 



