W. Koch and M. L. Koch 435 



in two instances where the value from one analysis only is pre- 

 ferred: Table IV gives the absolute weights of these constituents, 

 as found in one brain; while Table V gives the ratio of increase 

 of the different constituents, taking the amount of each constitu- 

 ent in the brain of the rat at birth to be unity and determining 

 the number of times each constituent had increased at successive 

 ages from birth to maturity. For comparison there is given in 

 Table II one analysis of the spinal cord at 120 days. 



DISCUSSION OF RESULTS. 



The growth of the nervous system from the first laying down 

 of the neural canal to maturity may be divided into four periods. 

 The first period, during which cell division is the most charac- 

 teristic feature, lasts to about birth. A short time before birth 

 cell division begins to decrease. The chemical changes during 

 this first period were not studied directly in the albino rat for 

 the reasons stated in the first paper 30 but the composition of the 

 nervous system in this primitive, undifferentiated state may be 

 seen in the analysis of the fetal pig brain reported in the first 

 paper of the series. At this time phosphatides are present, sul- 

 phatides are relatively less important and cerebrosides are en- 

 tirely lacking; proteins, phosphatides, extractives, salts and water 

 are the predominant constituents of the tissue. 



The second period (see Table VI) lasts from birth for about 

 ten days, when the third period begins. The second period is 

 characterized structurally by the development of fibers from the 

 cells and the increase in their size. Donaldson has estimated 

 that the number of nerve cells does not increase more than 3 to 

 6 per cent during this period, but the cells do add to the number 

 and size of their branching processes. This period, as may be 

 seen from Table VI, is one of intense growth of all the solid con- 

 stituents. The proteins continue throughout this period to be 

 formed at a very rapid rate, 4-5 mgms. being laid down per day. 

 Cerebrosides are either absent entirely, or present in very small 

 quantities. 



In the third period, that of most rapid growth, from the tenth 

 to the twentieth day, medullation begins. There is a wonderful 



30 Koch, M. L. : this Journal, xiv, p. 279, 1913. 



