138 THE CELL 



violet), and sometimes are more or less soluble (fuchsine, methyl 



orange, tropseolin). 



Further, animals afford us good examples of this storing up i 

 living cells. Fertilised eggs of Echinoidea acquire a more or less 

 intense blue colouration, if they are placed for a short time in 

 a very dilute solution of methylene blue (Hertwig IV. 12b). A 

 small accumulation of colouring matter does not arrest the process 

 of segmentation, which still continues, although somewhat slowly, 

 in a normal fashion, and in some cases may go on even until the 

 gastrula is formed. Here the colouring matter is chiefly deposited 

 in the endoderm cells, which points to the conclusion that it is by 

 the agency of the yolk material that the accumulation takes 

 place. Living Frog and Triton larvae become of an intense blue 

 colour if they are left for from five to eight days in a weak solution 

 of methylene blue. In this case the colouring matter combines 

 with the granules in the cells (Oscar Schultze V. 44). After 

 remaining for days in pure water they commence to become 

 colourless again. If indigo-carmine is injected directly into the 

 blood of a mammal, it is soon taken up both by the liver- eel Is and 

 by the epithelium of the convoluted tubules of the kidney, and 

 then is excreted either into the biliary ducts, or into the kidney 

 tubules (Heidenhain V. 42). If methylene blue is injected into 

 the blood, it combines with the substance of the nerve fibres, 

 imparting to them a dark blue colouration (Ehrlich V. 41). 

 Alizarin is stored up in the ground substance of the bones. 



Next to the chemical affinities, which exist between the par- 

 ticles of matter within the cell and those outside of it, the study 

 of the physical processes of osmosis is of the greatest importance 

 for the comprehension of the absorption and rejection of matter. 

 We must here observe whether the membrane, when present, is 

 more or less permeable. As a rule it is much more permeable to 

 dissolved substances than is the protoplasmic substance itself. 

 This latter is separated from the exterior by a peripheral layer 

 (cf. p. 15), which, according to Pfeffer, plays a most important 

 part in the process of osmosis. If some substance in solution is to 

 be taken up into the protoplasm, it must first be imbibed by the 

 peripheral layer ; that is to say, its molecules must become 

 deposited between the plasmic particles, and from there be trans- 

 ferred to the interior. Further, a substance in solution can, even if 

 it be not actually absorbed, produce an osmotic action by exerting 

 an attraction upon the water contained in the cell, and by thus 



