460 C. G. MACARTHUR AND E. A. DOISY 



GENERAL DISCUSSION 

 1 . Dominance of the nervous system 



There seem to be no facts presented in this paper that are 

 inconsistent with the theory that the chromosomes are very im- 

 portant in the early differentiation of cells. In fact, the data 

 suggest that nucleoproteins (tables 10 and 11, fig. 1) then phos- 

 phatids and simple extractive molecules dominate in the youngest 

 tissue. It is probable that there is a metabolic gradient in the 

 fertilized cell that is important in determining which will be the 

 head end of the organism (Child, '12). Very early in growth 

 nervous tissue differentiates in this head region. Because of 

 this early formation, much of the later development in other parts 

 of the body is rather dependent on the nervous system. It needs 

 to be emphasized that this dominance of one substance over 

 another (or one organ over another) is but relative. Most of 

 them are developing together, but their influence on each other 

 is very different. Very probably growth consists in the forma- 

 tion of continually larger quantities of the respective cell constit- 

 uents in a more or less definite order, commencing with the 

 nucleoproteins (table 11, fig. 2) of the chromosomes. In the 

 brain there is rapid formation of certain substances, then the 

 presence and formation of the substances influence the rate of 

 formation of another substance, this another, and so on till all 

 have come into adjustment with the new conditions. While 

 these changes are occurring in the brain, and partly because of 

 them, other areas of differentiation are split off, which are to 

 become other organs. Then in these areas a similar cycle of 

 changes will occur. Probably each of the organs has periods of 

 maximum growth. When such unusually large amounts of ma- 

 terial are being formed rapidly, in comparison with the rate of 

 formation at other periods of growth, we get an irregularity in 

 the main curve of growth that produces a so-called growth cycle. 



During growth substances are regularly coming to the various 

 organs through the blood. We do not now believe that the 

 amount of either these food substances or the oxygen de- 

 termines the rate of growth, though they do influence it. The 



