288 RESEARCHES ON CORTICAL LOCALISATION AND 



birth and in early infancy it is still little more than one-half of the 

 adult depth. 



Lamina 3, Granule Layer, Middle Cell Layer. The granule layer 

 develops in the sixth month of fo3tal life, and at this period it is 

 separable from the super] acent outer cell or pyramidal layer owing 

 to the more differentiated condition of its cell elements. At this 

 period it is already one-half of the adult depth, and by the time 

 of birth it has attained to a depth which is nearly three-fourths 

 of this. 



Lamina 5, Polymorphic Layer, Inner Cell Layer. The poly- 

 morphic layer is the first cell layer to appear, and it is separated 

 off from the rest of the partially differentiated neuroblasts of the 

 cortex by the development of the fourth or inner fibre lamina at 

 the sixth month of fo3tal life. The polymorphic layer is already, 

 at the period referred to, about three-fourths of the adult depth, 

 and it undergoes a slow further development until after birth. In 

 a child of six weeks it has attained a depth which is within 18 per 

 cent, of the adult normal. 



PRIMARY FIBRE LAMINA OF THE CORTEX 



Lamina 1, Superficial Layer, Outer Fibre Layer. At the fourth 

 month of foetal life the cortex consists of this layer and of a 

 deeper undifferentiated mass of neuroblasts. The layer under 

 description is already about one-half of the adult depth, and it 

 remains unchanged until the development of lamination in the 

 sixth month. At birth, however, it has attained to a depth which 

 is about two-thirds of the adult normal. It is probable that its 

 further development to the normal adult depth occurs in associa- 

 tion with that of the subjacent second, pyramidal, or outer cell 

 layer. 



Lamina 4, Inner Fibre Layer, Inner Line of Baillarger. This 

 layer appears in the sixth month of foetal life, and almost at once 

 attains to nearly the normal adult depth. The cleavage of the 

 partially differentiated neuroblasts of the cortex into an upper and 

 a lower portion by the development of this layer is the most striking 

 feature of the process of lamination. In view of what will be 

 stated later concerning the functions carried on by the inner cell 

 layer, this cleavage of the neuroblasts is an occurrence of the 

 greatest significance. 



