Burrows: Study of Body Cells. 477 



birth. Subsequent to this time no new oi-^an or parts of organs are 

 formed except the hiying down of the nerve sheaths. All subse- 

 quent growth is merely the expansion of previously formed struc- 

 tures. It is like hypertropliy and hyperplasia, as they result from 

 stimulation of the adult organs. 



With the completion of the building of the organs, function has 

 made its appearance. Growth unlike that of the earlier period now 

 runs hand in hand with new forms of work. This growth continues 

 to maturity, when it ceases, except as it is to take care of the wear 

 and tear, so to speak, and to play a part in certain organs and tis- 

 sues, such as the bone marrow, the sex glands, and the nail- and 

 hair-beds. Atrophy then slowly intervenes. This continues to the 

 inevitable death. 



The general nature of the structure and the metabolism which 

 leads to this sequence of changes in the organism is the pertinent 

 biological question to-day. ChikU in 1915 reviews the general the- 

 ories that have been advanced and attempts to give a physicochemi- 

 cal explanation of the process. Child, appreciating the relation 

 between the slowing of growth and the decrease in metabolism, 

 makes the general assumption, applying the "law of mass action," 

 that the decrease in metabolism may be explained as the result of 

 the gradual accumulation within the cell of an insoluble substratum. 

 Since differentiation and the development of function takes place 

 hand in hand with this slowing of metabolism, he looks upon them 

 also in the same light. He considers the changes in the structure — 

 differentiation — the result of the same slow deposit of the same sub- 

 stratum. This substratum he conceives as one of the products of 

 the metabolic reaction. The reaction then, like any incomplete re- 

 versible physicochemical reaction, becomes slowed and this sub- 

 stratum accumulates, and ceases when it reaches a certain con- 

 centration. Death is the equilibrium point for this reaction. 



While there is little doubt that many heterogeneous, like many 

 homogeneous, reacting systems reach their equilibrium by this route, 

 there does not seem to be sufficient evidence to show that the body is 

 exactly of this kind. Again, all such systems which do arrive at 

 such an equilibrium are first put together by some external force. 

 In nature one cannot obtain more energy from any system than what 

 has been put into it. This is the law of the conservation of energy. 

 .Child looks upon rejuvenation, then, as the result of the removal of 

 this substratum and of dedifferentiation. Differentiation is not, there- 

 fore, according to Child, a change peculiar to a definite period, but 



