ELEMENTARY VITAL PHENOMENA 165 



puted more than twenty years ago by Pfliiger (75, 1). As a matter 

 of fact, as the above consideration has shown, the difference consists 

 only in that the plant-proteid of the chlorophyll-bodies has re- 

 tained from early times the property of assimilating inorganic 

 material, while animals require for the construction of their living 

 substance organic food-material already prepared. Nevertheless, 

 synthetic and analytic processes take place in both the plant and 

 the animal body. In the plant the decomposition of carbonic 

 acid must precede the synthesis of starch ; in order that the starch 

 may be further elaborated, it must first be decomposed into simple 

 kinds of sugar, and so on. Finally, in the plant also there occurs 

 the whole series of cleavages that are associated with the decom- 

 position of the proteid molecule, with dissimilation, exactly as in 

 the animal body. But syntheses take place in the animal body 

 to a great extent. The further elaboration of the digested pro- 

 teids, fats, and carbohydrates towards the construction of living 

 substance involves extended synthetic processes, and it has been 

 seen that the majority of the products of retrogressive proteid- 

 metamorphosis are formed synthetically out of the cleavage- 

 products of the proteids. Hence analytic and synthetic processes 

 go hand in hand in the animal- as in the plant-cell, and the old 

 distinction into analytic and synthetic organisms is merely the 

 expression of an earlier stage of our knowledge of the chemical 

 processes in living substance. 



C. THE OUTPUT OF SUBSTANCES 



Living substance excretes transformation-products in the same 

 proportion in which it receives substances from the outside and 

 transforms them ; the substances given out are as varied as those 

 taken in. But with our slight knowledge of the transformations 

 und with the overwhelming number of substances excreted by the 

 various forms of cells, we can say in a very few cases only by what 

 processes the substances are derived. As regards most of them, it 

 is not known whether they are derived from assimilatory or dis- 

 similatory transformations ; evidently a large quantity of by- 

 products are formed in both the ascending and the descending 

 portions of the metabolic series, whether by simple cleavage, or 

 by synthesis from the cleavage-products or other substances 

 which are excreted by the organism either for some further use or 

 as useless products. This last point, as to whether the excreted 

 substances are of still further use in the life of the organism, or 

 are removed as useless products, as slag, has caused a distinc- 

 tion to be recognised among the substances given off. Although 

 it is difficult to make this distinction sharp, because of the ex- 

 traordinary variety of different products, the use of it is advisable 



