364 Journal of Comparative Neurology. 



tion from its ental to its ectal portion. This process continues 

 until the medullated fibers of the II and III Meynert layer 

 meet those of the zonal layer. ^ The final stage in the develop- 

 ment of the cortex is the addition of a secondary system of 

 fibers to the primary system just described. The secondary 

 system is entirely composed of coarse fibers. They are first 

 seen scattered throughout the outer association layer. Shortly 

 afterward the same sort of fibers are found in the II and III 

 Meynert layer — at first singly, and later organized into a thin 

 stratum which Kaes calls the '• Bechterew streak. " Still later 

 very coarse fibers appear also in the Baillarger layer, and in 

 the most highly developed cortex known, they are found as a 

 secondary system of coarse fibers extending from the ental to 

 the ectal border of the cortex.^ Very few regions of the cortex 

 reach this highest stage even in the completely developed adult. 

 The period from youth to maturity is in general characterized^ 

 by the gradual growth of the II and III Meynert layer and the 

 formation of the secondary fiber systems, but even in the fully 

 developed brain there are regions which do not reach the stage 

 at which the II and III Meynert layer exhibits medullated 

 fibers. 



The final stages of development reached by any region 

 may be estimated in either of two ways: either by the thick- 

 ness of the cortex as a whole, and of its constituent layers ; or 

 by the complexity of its medullated systems. The determina- 

 tion of the degree of development by cortical thickness is some- 

 what complicated by a fact to be dwelt upon later, the fact that 

 during the growing period of youth, a thinner cortex means 

 greater development, but that after the cessation of body- 

 growth a thinner cortex means less development. The 

 point at which this change occurs for those regions which 

 reach the highest development, is about eighteen years, 

 but for the less advanced regions, thinning of the cortex 



1 3. p. 42. 



* 6, p. 11-12 ; 4, p. 2; 8, p. 4. 



•6. p. 5. 



