GROWTH OF THE CEREBRAL CORTEX 585 



must however increase in area, as the hemispheres enlarge. 

 Mitosis having almost ceased at the end of the second" phase, this 

 increase in area must be due mainly to an increase in the diame- 

 ters of the cell bodies and to the increase in the number and the 

 size and the myelination of the fibers. This increase in area 

 after the end of the second phase is very considerable. When 

 the weight of the brain increases from 1.15 grams to 2.03 grams 

 (table 14), the area increases some 46 per cent and it is this ex- 

 tension of area which is accomplished by the cortex, after growth 

 in thickness has come nearly to an end. A detailed study of the 

 manner in which this extension is accomplished must be reserved 

 for another occasion. A word may be said however regarding 

 the age relations of myelination in the rat's brain. 



In his study on the myelination of the central nervous system 

 of the albino rat, Watson ('03) found in the cerebrum the first 

 myelination or investment of the axons with myelin sheaths, as 

 indicated by the substance which stains with the Weigert-Pal's 

 method, to begin at the ages given below. 



Age of the beginning 

 Localities in cerebrum of myelination 



Capsula externa 11th day. 



Stria olfactoria lateralis 14th day. 



Corpus striatmn 14th day. 



Corpus callosum 14th day. 



Radiation into the cortex 14th day. 



Commissura anterior 17th day. 



Thalamus . .' 17th day. 



The fibers radiating into the cortex myelinate very slowly, 

 however, so that but few are to be seen till after twenty-fifth 

 day. This age, the twenty-fifth day, would correspond to the 

 early part of the third phase. 



The fact that the cortex attains nearly its full thickness before 

 the radiating fibers are myelinated should mean that the organiza- 

 tion of the cortex occurs while growth in thickness is in progress. 

 After this organization has been made, then myelin begins to 

 appear around the axons, increasing their diameter. The in- 

 crease in cortical area during and after the third phase must, 

 therefore^ be caused principally by the enlargement of the cell- 



