Thompson, Medullation of Cortical Fibers. 363 



completed, two other sets of fibers appear. One of them is 

 found surrounding the outer limit of the projection fibers, which 

 by this time have completed the normal extension of their 

 medullation in the cortex. This is the Baillarger or Gennari 

 layer. It marks the ectal border of the outer association layer, 

 but is composed of coarser fibers than the rest of the layer. ^ 

 The second set of fibers which appears at this period is the 

 zonal layer, at the ectal border of the cortex.^ It also is com- 

 posed of coarse fibers. It is the stage of development just 

 described which is characteristic of the advanced portions of 

 the cortex of the child of a year and a quarter. Its distinctive 

 features are, in brief, a partly developed outer association layer, 

 consisting of some stratified fibers lying next the fibrae propriae, 

 and the Baillarger layer at its ectal border. These two portions 

 of the outer association layer are separated by a region where 

 the fibers of the layer are yet unmedullated. The Baillarger 

 layer is separated on the other side from the zonal layer, by a 

 space devoid of medullated fibers — the region of the future II 

 and III Meynert layer. 



As the process of development continues, the fibers of the 

 outer association layer spread gradually toward the Baillarger 

 until they reach it.^ At the same time the Baillarger and zonal 

 layers grow thicker and richer in fibers. The next stage is 

 marked by the appearance of the inner Baillarger layer and the 

 first fibers of the II and III Meynert layer. The inner Bail- 

 larger layer appears as a narrower line of coarser fibers among 

 the finer fibers of the outer association layer, just ental to the 

 outer Baillarger layer.* The fibers of the II and III Meynert 

 layer are the finest of the cortex. The first of them to become 

 medullated are those lying next to the outer Baillarger layer. 

 The general course of development of this layer is, like that of 

 the outer association layer, a gradual spreading of the medulla- 



1 S. p. 45; 8, p. 3; 6, p. II. 

 >6, p. 11; 8, p. 3. 



' 5. P- 45- 

 *6, p. II. 



