70 THE DATA OF BIOLOGY. 



substance which, in each of the cells generating it, is con- 

 tained in a plexus of protoplasmic threads: again a nitro- 

 genous body diffused through a mass which is now formed 

 out of sugar and is now dissolved again into sugar. For it 

 appears that this soluble form of carbo-hydrate, taken into the 

 liver from the intestine, is there, when not immediately 

 needed, stored up in the form of glycogen, ready to be re-dis- 

 solved and carried into the system either for immediate use 

 or for re-deposit as glycogen at the places where it is pre- 

 sently to be consumed: the great deposit in the liver and 

 the minor deposits in the muscles being, to use the simile of 

 Prof. Michael Foster, analogous in their functions to a central 

 bank and branch banks. 



An instructive parallelism may be noted between these 

 processes carried on in the animal organism and those car- 

 ried on in the vegetal organism. For the carbo-hydrates 

 named, easily made to assume the soluble or the insoluble 

 form by the addition or subtraction of a molecule of water, 

 and thus fitted sometimes for distribution and sometimes for 

 accumulation, are similarly dealt with in the two cases. As 

 the animal-starch, glycogen, is now stored up in the liver or 

 elsewhere and now changed into glucose to be transferred, 

 perhaps for consumption and perhaps for re-deposit; so the 

 vegetal starch, made to alternate between soluble and in- 

 soluble states, is now carried to growing parts where by 

 metabolic change it becomes cellulose or other component of 

 tissue and now carried to some place where, changed back 

 into starch, it is laid aside for future use; as it is in the 

 turgid inside leaves of a cabbage, the root of a turnip, or the 

 swollen underground stem we know as a potato : the matter 

 which in the animal is used up in generating movement and 

 heat, being in the plant used up in generating structures. Nor 

 is the parallelism even now exhausted; for, as by a plant 

 starch is stored up in each seed for the subsequent use of the 

 embyro, so in an embryo-animal glycogen is stored up in the 



