352 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



mantle, and marginal layers. During the first two months the cortex remains 

 thin and differentiation is slow. At eight weeks neuroblasts migrate from the 

 ependymal and mantle zones into the marginal zone and give rise to layers of 

 pyramidal and other cells typical of the cerebrum. The differentiation of these 

 layers is most active during the third and fourth months, but probably continues 

 until after birth (Melius, Amer. jour. Anat., vol. 14, 1912). From the fourth 

 month on, the cerebral wall thickens rapidly, owing to the development of (1) the 

 fibers from the thalamus and corpus striatum; (2) of endogenous fibers from the 

 neuroblasts of the cortex. The fibers form a white, inner, medullary layer sur- 

 rounded by the gray cortex. Myelination begins shortly before birth (Flechsig), 

 but some fibers may not acquire their sheaths until after the twentieth year. As 

 the cerebral wall increases in thickness the size of the lateral ventricle becomes 

 relatively less, its lateral diameter especially being decreased. 



Anomalies. — Defects involving the neural tube may give rise to various monstrous 

 conditions, e. g., cyclopia, acrania. In spina bifida, a sac-like protrusion of the cord, or its 

 membranes, extends through a cleft in the vertebral column. 



