10 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



the chief constituent of a substance which, in its primary uu- 

 modifiecl state, is known diS protoplasm. 



2. Its universcd disintegration and waste by oxidation; 

 and its concoinitant reintegration hy the intussusceptioii of 

 new matter. 



A process of waste resulting from the decomposition of 

 the molecules of the protoplasm, in virtue of which they 

 break up into more highly-oxidated products, which cease to 

 form any part of the living body, is a constant concomitant 

 of life. There is reason to believe that carbonic acid is al- 

 ways one of these waste products, while the others contain 

 the remainder of the carbon, the nitrogen, the hydrogen, and 

 the other elements which may enter into the composition of 

 the protoplasm. 



The new matter taken in to make good this constant loss 

 is either a ready-formed protoplasmic material, supplied by 

 some other living being, or it consists of the elements of 

 protoplasm, united together in simpler combinations, which 

 consequently have to be built up into protoplasm by the 

 agency of the living matter itself. In either case, the addi- 

 tion of molecules to those which already existed takes place, 

 not at the surface of the living mass, but by interposition 

 between the existing molecules of the latter. If the processes 

 of disintegration and of reconstruction which characterize 

 life balance one another, the size of the mass of living matter 

 remains stationary, while, if the reconstructive process is the- 

 more rapid, the living body grows. But the increase of size 

 which constitutes growth is the result of a process of molec- 

 ular intussusception, and therefore differs altogether from the 

 process of growth by accretion, which may be observed in 

 crystals and is effected purely by the external addition of 

 new matter — so that, in the well-known aphorism of Linnseus,' 

 the word "grow," as applied to stones, signifies a totally dif- 

 ferent process from what is called "growth " in plants and 

 animals. 



3. Its tendency to undergo cyclical changes. 



In the ordinary course of Nature, all living matter proceeds 

 from preexisting living matter, a portion of the latter being 

 detached and acquiring an independent existence. The new 

 form takes on the characters of that from which it arose ; ex- 

 liibits the same power of propagating itself by means of an 

 offshoot ; and, sooner or later, like its predecesjsor, ceases to 



^ " Zap/^e,s crescunt : v^^e^a&iZe'a crescunt et vivunt: a«mo?«a crescunt, vi- 

 vunt et sentiunt." 



