352 THE MORPHOGENESIS OF THE CENTRAL' NERVOUS SYSTEM 



the cortex remains thin and differentiation is slow. At eight weeks, 

 neuroblasts migrate from the ependymal and mantle zones into the mar- 

 ginal 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 

 (Mellus, 1912). From the fourth month on, the cerebral wall thickens 

 rapidly, owing to the development: (i) of 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 surrounded 

 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. There are numerous types of defective neural tube development most 

 the result of arrest. These usually involve the bony investments as well, and produce 

 conspicuous malformations. 



The more or less extensive failure of the neural groove to close produces cranioschisis 

 (acrania), or rachischisis, depending on whether, the region of the head or vertebral column 

 is affected. If the cleft contains a sac-like protrusion of the membranes, the condition is 

 known as meningoccele; if the neural wall alone protrudes, it is encephaloccele (brain) or 

 myelocoele (spinal cord) ; if, as is most common, both are involved, it is meningo-encephalo- 

 ccele, or meningo-myeloctele. Such a hernial condition of the spine is often called spina 

 bifida and is most frequent in the lumbo-sacral region, where the sac may become the size 

 of a child's head. 



An excessive fluid content in the, brain cavities causes both brain and skull to enlarge, 

 producing hydrocephaly. 



