730 CELL PENETRATION BY ACIDS. V 



penetrability of the mantle epithelium of Chromodoris toward acids 

 may be caused to vary in the direction either of increase, or of dim- 

 inution, while the spontaneous diffusion of pigment (uncomplicated 

 by muscular contractions, in this instance,) simultaneously changes 

 in a directly contrary manner. The pigment involved serves as 

 indicator of internal acidity and is at the same time the freely 

 water-soluble coloring matter concerned in visible outward dif- 

 fusion. For comparative tests it is necessary to use tissues colored 

 in precisely the same way by this pigment. 



min. 



Experiment 8.34.3. — (c) Two pieces of mantle placed in 0.1 n HCl; pene- 

 tration time (24°) 2 . 16 



2.05 



(h) Two similar pieces placed in sea water one-half saturated with CHCI3, 



for 5 minutes; then put into 0.1 n HCl; penetration time (24°) 2 . 25 



3.75 



In (a) loss of pigment was evident in 4 and 10 minutes, respectively; in {h), 

 within 2 minutes in both cases. 



E.vperimenl 8.42.1. — Four pieces of mantle tissue from a single individual 

 were used; two were placed in 0.1 N HNO3, the penetration times being 3.25 and 

 3.50 minutes, respectively; two others were placed for 10 minutes in sea water 

 containing 0.1 M urea, then transferred to 0.1 n HNO3 — the penetration times 

 being now 0.5 and 0.3 minutes. In the first case pigment diffusion was observed 

 in 7 minutes, in the second in 9 minutes. 



These tests, typical of a number made, show clearly that the 

 condition of the tissue used may differ markedly when regarded from 

 the standpoints (1) of penetrability for acids, and (2) of the readiness 

 with which spontaneous loss of pigment occurs. A long series of 

 observations upon the penetration of various acids showed, in addi- 

 tion, that practically no correspondence obtains between the relative 

 ease of penetration and the speed with which pigment is lost to the 

 surrounding solution. With mineral acids, pigment loss in general 

 followed the internal color change indicative of penetration; while 

 with weak acids, and especially with fatty acids, even in the case of 

 relatively concentrated solutions penetrating with high velocity, 

 loss of pigment usually preceded the internal sign of acid penetration. 

 Quite apart from the fact that it seems inadvisable to speak of 

 "permeability" as a general property, having reference indis- 



