DEVELOPMENT OF THE XEKVOUS SYSTEM 475 



thalamus, the cerebral portion of the pituitary gland, the pineal 

 gland, and the corpora mamillaria. 



3. From the Mesencephalon. The corpora quadrigemini, the 

 crura cerebri (pedunculi cerebri), and the aqueduct of Sylvius. 



4. From the Metencephalon. The isthmus rhombencephali, 

 pons Variolii, and cerebellum. 



5. From the Myelencephalon. The medulla oblongata. 



The Ependyma. The epiblastic cells which line the lateral 

 walls of the slit-like neural canal become elongated or columnar in 

 shape, and are called ependyma cells. Many of these are germinal 

 cells, exhibiting the various stages of mitosis ; the resulting cells 

 promptly differentiate into two distinct varieties, the spongioblast 

 and the neuroblast. 



The spongioUasts are long cells whose expanded bases line the 

 neural canal and whose elongated bodies are directed outward 

 toward the surrounding mesoblastic tissue. The interlacing proc- 

 esses of these cells, together with those from the neuroblasts, form 

 a network or neurospongium. The spongioblasts are destined to 

 develop the neuroglia. 



The neuroblasts are small ovoid or fusiform cells, which, though 

 usually bipolar, very early develop a long peripheral process 

 directed away from the neural canal ; this process is the primitive 

 neuraxis. In those cells which are to form the nerve roots and 

 the peripheral ganglia this process grows outward into the sur- 

 rounding mesoblast, the cell gradually migrating along the same 

 course, but leaving in its wake the developing central process 

 which in some cases is dendritic in character, but in others is des- 

 tined to form a second centrally directed branch of the neuraxis. 



It is by means of this property of locomotion that these cells 

 reach their destination in the various cell groups or nuclei in the 

 brain and spinal cord or in the peripheral ganglia. In this way 

 the fibre paths or tracts of the central nervous system as well as 

 the cranial and spinal nerves and their ganglia are formed. 



Since, during the period that the neuroblast is performing this 

 migration, it is also developing its neuraxis and dendrites, by the 

 time it reaches its permanent location its principal portions, cell 

 body, neuraxis, and dendrites are already formed and are ready to 

 functionate. The further history of the neuroblast is merely one 

 of continued growth, pushing forward its neuraxis to still more 

 distant and more complex relations. 



