686 NAOKI SUGITA 



bodies, the increase in the diameter of the axons and the forma- 

 tion and enlargement of the myehn sheaths. 



This conclusion is supported by the table 74 in ''The Rat" 

 (Donaldson, '15) which gives the percentage of water in the al- 

 bino rat brain. From the values in that table, the mass of dry 

 substances has been calculated and results are plotted according 

 to brain weight in chart 12. In the first phase of the cortical 

 development, the solids of the brain increase proportionally to 

 the brain weight, but after the middle of the second phase (brain 

 weight 1.00 grams) and through the third phase, they increase 

 much more rapidly, again decreasing in rate at about 35 days 

 after birth (brain weight 1.40 grams). This means clearly that 

 from the middle of the second phase (about 15 days of age) some 

 new substances begin to be deposited rapidly in brain, i.e., myeli- 

 nation is in progress. Looking at chart 26 given in "The Rat" 

 (Donaldson, '15), we see that the percentage of water in the 

 brain decreases comparatively slowly during the first ten days 

 after birth, then, from the tenth day, decreases rapidly till the 

 thirtieth day, after which the decrease becomes slow again. At 

 the sixtieth day after birth, the brain (weight 1.60 grams, per- 

 centage of water ca. 79 per cent) has reached nearly to the 

 stage of the adult in percentage of water. This indicates that 

 myelination is going on energetically between the tenth and the 

 thirtieth day after birth. The myelination in the cortical radia- 

 tion begins late, accelerating its rate after the twentieth day. 



Bringing together the foregoing observations and inferences, 

 we may make the following statements concerning the growth 

 changes in the cerebral cortex during the three phases which 

 have been recognized. 



The first phase covers the first ten days after birth. This 

 phase represents the period in which the thickness of the cortex 

 increases rapidly by means of both cell immigration and multi- 

 plication and cell enlargement. If the data of the Group I 

 (tables 1, 3, 5, 6, 8, 9) from rats before normal birth are also 

 taken into consideration, it is evident, as shown by the first 

 entries on Charts 1, 3, 4, 5, 6 and 9, that the increase of the 

 cortical thickness is equally rapid just before birth. 



