368 Journal of Comparative Neurology. 



fibrae propriae. Thus the ectal border of the fibrae propriae — 

 which is also the ental border of the cortex — is shifted toward 

 the surface of the gyrus, and the portion measured as cortex 

 grows thinner, while the medullary center grows broader. This 

 is the general nature of the process by which the reduction is 

 cortical thickness from childhood to youth takes place. When 

 we turn attention to the details of the process and inquire what 

 changes occur in the separate layers, we are met at the outset 

 by the statement that the outer association layer of the child is 

 of about the same thickness as that of the youth, while the II 

 and III Meynert layer is several times as thick. The explana- 

 tion offered for this relationship rests primarily upon the method 

 of determining the boundary between these two layers at dif- 

 ferent stages of development. As we have said in the general 

 description of the course of development, the first of the intra- 

 cortical fibers to become medullated are the most ental fibers 

 of the outer association layer. They appear as finer fibers 

 forming a brief continuation of the fibrae propriae toward the 

 surface of the cortex. Very early this layer of fibers reaches 

 a thickness approximately equal to that of the outer association 

 layer of the youth. Now as development proceeds, the spread- 

 ing of the medullary center already described pushes the ectal 

 border of the layer of fibrae propriae towards the surface of the 

 cortex, while at the same time the ectal border of the outer 

 association layer continues to move toward the surface because 

 of the gradual medullation of its fibers. Thus the ental and 

 ectal borders of the outer association layers move simultane- 

 ously in the same direction, and we observe the phenomenon 

 of a shift of the entire outer association layer toward the sur- 

 face of the cortex, without any appreciable change in the thick- 

 ness of the layer itself. But during this process, all the dis- 

 tance between the ental border of the zonal layer and the shift- 

 ing ectal border of the outer association layer is measured as 

 the II and III Meynert layer. 



The consequence of this procedure is that we have at first 

 a very diproportionately broad II and III Meynert layer, which 

 is gradually reduced in thickness by the shifting of the outer 



