l] ASSIMILATION 5 



tions. Thus, all the hard skeletons of animals are really insoluble 

 excreta. On the other hand, the gastric juice which digests the 

 food in the human stomach, and the slime or mucus, which 

 prevents a frog from drying up when taken out of water, are fluid 

 excreta. A part of the body specially adapted to produce a secre- 

 tion is termed a gland. 



Other products of decomposition reconstitute, as we have seen, 

 the original molecule by combining with the necessary elements 

 from the food; this process is known asanabolism (Gr. ava.pd\Xeiv, 

 to put back or up) or assimilation. Inasmuch as, generally speak- 

 ing, from the breaking up of one molecule more than one residue is 

 produced capable of regeneration, there is an increase in the number 

 of biogen molecules causing an increase in bulk of the protoplasm, 

 or growth 1 . 



It is believed that both biogen and proteid molecules are of the 

 nature of compound amino-acids. An amino-acid is an acid in which 

 not only is the place of the central acid radicle corresponding to 

 the sulphur in sulphuric acid or the phosphorus in phosphoric acid 

 taken by a group of atoms containing carbon but this carbon group 

 contains also NH 2 the radicle of ammonia. As a consequence an 

 amino-acid can act not only like an acid in combining with an 

 alkali but like an alkali combining with an acid it is both an acid 

 and a compound ammonia and has in consequence two hands 

 and owing to this circumstance it can combine with another group 

 similar to itself by uniting so to speak its acid hand with the 

 alkaline hand of the new group or vice-versa and so build up a 

 complex chain. 



The regeneration of the biogen takes place at the expense of the 

 food. Taking in food is called eating, or ingestion. Since, how- 

 ever, the food must penetrate to every portion of the protoplasm 

 it must be dissolved a process effected by the chemical action 

 of certain products of the decomposition of the biogens, known 

 as ferments. The process is called digestion. The products of 

 digestion must be assimilated ; in order that this may be accom- 

 plished they are decomposed until quite simple substances are 

 formed ; in a word the amino-acid chain constituted by the ingested 

 proteid is broken into its individual links. The casting out of an 

 insoluble remnant of the food is called defaecation, and inasmuch 

 as such remnants have never formed part of the biogen molecule, 



1 See Verworn, General Physiology (Engl. Edition), 1899, p. 486. 



