W. Koch and M. L. Koch 441 



outburst of activity in forming phosphatides which reach a max- 

 imum rate of formation of 3.5 mgms. per day. This change is 

 no doubt correlated with the great growth of the fibers and the 

 beginning of medullation. The organic extractives and inorganic 

 constituents continue to be formed at the same rate since the 

 cell bodies are increasing in size; probably not more than from 

 10 to 20 per cent having reached anything approaching adult 

 size up to this time. 



The sulphatides, although present in less quantity than the 

 phosphatides, reach also their maximum rate of formation. The 

 whole chemical picture is that of a rapid growth of protoplasm, 

 with a change in its character owing to the increase of phospha- 

 tides. During this period the neurones increase rapidly in size. 



Attention is particularly directed to the temporary great in- 

 crease in neutral sulphur during these two periods of intense 

 growth (Table IV). The significance of this has already been 

 discussed on p. 431 et seq. 



The fourth growth period is the period of continued medulla- 

 tion. This period is characterized chemically by a great reduc- 

 tion in the rate of formation of all substances except the cerebro- 

 sides. These latter between 20-40 days come into view, almost 

 equalling the phosphatides and being more than half the amount 

 of the proteins formed at the same time. The cerebrosides con- 

 tribute a large share toward medullation. The rate of formation 

 of the various constituents per day falls in the 20-40 day period, 

 as compared with the 10-20 day period, in the case of the proteins 

 to one-seventh; the phosphatides to one-fifth; the sulphatides to 

 one-third; and the organic and inorganic extractives to one-third. 

 The formation of the proteins decreases the most; the cerebrosides, 

 the least. If the rate of formation in the 40-120 day period is 

 compared to that of the 10-20 day period it is seen that the 

 protein formation has decreased to one-sixteenth; the phospha- 

 tides to one-twentieth; the sulphatides to one-fourth but are still 

 increasing. On the other hand, the organic extractives and inor- 

 ganic constituents have not increased at all, indicating that metab- 

 olism is much reduced in its rate and the growth of the protoplasm 

 is much slower. During this fourth period then, the sulphatides 

 continue to be formed at a more rapid rate, relative to their 

 total amount, than any other constituents; and in the 120-210 



