Burrows: Study of Body Cells. 499 



has to do with differentiation is their inability to utilize the crude 

 food materials supplied by nature. This deduction we are forced 

 to believe, however, not only from the above observations, but from 

 a host of already carefully accumulated facts. If the first two 

 blastomeres of many lower forms are separated, two animals de- 

 velop. The yolk is also separated by this process. Each animal is 

 but one-half the size. Gudernatsch has shown that tadpoles fed 

 upon thyroid differentiate within a few days, while thymus feeding 

 delays this process at least for a very long time. Very large tad- 

 poles may be thus developed. The ultimate heterogenicity thus de- 

 veloped is essential for subsequent life and for reproduction. The 

 egg cell builds a new system like the old (heredity), in that it has 

 acquired from the old a supply of those substances necessary to 

 carry it through the building of the new system, which is again ca- 

 pable of preparing these substances from the crude materials avail- 

 able in nature wdthout. The building results from the changing 

 syntheses which result from the gradual decreasing yolk supply. 

 The form of the building is dependent, therefore, primarily on the 

 original constitution of the yolk. This is again dependent abso- 

 lutely on the nature of the machine which produces it. Each ani- 

 mal must, therefore, in each case reproduce its kind. The problem 

 of heredity is thus reduced to pure physics and chemistry. 



Death in such a system may result, therefore, from the destruc- 

 tion of essential parts, or the inevitable equilibrium of those forces 

 which maintain the heterogenicity. This does not mean a funda- 

 mental change in the cell. The cell succumbs as the result of this 

 breakdown. There is no reason why any of these cells may not 

 grow actively again if the organization is changed about it. Differ- 

 entiation is both chemical and mechanical in nature. Chemical dif- 

 ferentiation does not effect the fundamental energy-producing reac- 

 tion of the fixed tissue cells. It is an indication of the chemical 

 heterogenicity of the mature organism. The mechanical differen- 

 tiation essential for the dynamic state is reversible. According to 

 the above formulation, mechanical differentiation in early life is the 

 result of the removal and decrease in certain materials of the yolk. 

 If a change in organization suitable for active growth should take 

 place in the organism, no such differentiation should follow. The 

 cells suffering such changes must continue to grow as long as the 

 body supplies the necessary substances. The body must supply 

 those substances necessai-y for active growth as long as it survives. 



