588 NAOKI SUGITA 



The third phase begins at the twentieth day after birth and 

 lasts to the ninetieth day after birth when the rapid growth of 

 body decreases. During and after this phase the cortical thick- 

 ness remains almost fixed, despite the increase in brain weight or 

 brain size. Rapid myelination of the cortical fibers first appears 

 at the beginning of this phase and is also present in the corpus 

 striatum or in other central nuclei. Thus the increase in brain 

 mass during this phase is due principally to the deposition of 

 the myelin sheaths. The fact that the cortex has reached its 

 full thickness before the rapid development in myelination 

 begins, leads us to conclude that the formation of the myelin 

 sheath on any axon does not begin until the axon has estab- 

 lished functional connections with one or more other neurons. 

 During subsequent growth, both the axon and its myelin sheath 

 lengthen and enlarge without disturbing their relative volume 

 relations. It appears that the very considerable increases in the 

 area of the cortex, after its thickness is attained, is accom- 

 plished largely by the formation of the myelin sheaths. 



X. SUMMARY 



1. The postnatal increase in the thickness of the cerebral 

 cortex according to body growth has been systematically inves- 

 tigated, employing as material 96 male and 28 female albino 

 rats, representing every stage of postnatal life. 



2. The material was fixed, imbedded and stained by a uni- 

 form technique devised for the present investigation. The cor- 

 tical thickness was measured on sagittal, frontal and horizontal 

 sections, 10 micra thick, all of which were taken from the fixed 

 levels of the brain. Two brains were required for the three 

 sections. On these sections, thirteen localities in all were meas- 

 ured. These localities were accurately determined and repre- 

 sent regions well distributed over the surface of hemisphere. 

 The values thus obtained from the measurements on the slide 

 were later converted into the actual thickness of the cortex in 

 fresh condition, by the use of correction-coefficients based on 

 observation. This series of determinations is the first we have 



