THE NERVOUS SYSTEM. 



491 



differentiate into nerve cells as described below. Still others probably migrate 

 outward as indifferent cells, which later proliferate and form cells which differ- 

 entiate into neuroglia and nerve cells. 



According to recent researches (Cajal), by means of the silver stain of Cajal 

 the first indication of the differentiation of cells into nerve cells is the appear- 

 ance of neurofibrils in the cytoplasm of cells near the lumen. The part of the 

 cell in which the neurofibrils first appear is called the fibrillogenous zone 

 (Held) and is usually in the side furthest from the lumen. The cells in which 

 these appear are apparently without processes, and are accordingly termed 

 apolar cells (Cajal). (Fig. 424.) 



FIG. 424. Section through the wall of the fore-brain vesicle of a chick embryo of 3^ days. Cajal. 



A, b and c, Differentiating nerve cells in apolar stage, the neurofibrils are black; a, cell in a stage 

 transitional to the bipolar stage; B, bipolar cells; c (at lower right corner) , cone of " growth " 

 of developing axone; e, tangential axone. The cells in the bipolar stage have migrated out 

 ward, but 'the neuroblast or mantle layer has not yet been differentiated. 



The next step in the development of many, but probably not all, of these cells 

 is their transformation into bipolar cells by the outgrowth of two neurofibrillar 

 processes, one directed toward the lumen, the other, usually thicker, toward the 

 periphery, the cell body at the same time beginning to migrate outward (Fig. 424). 

 This bipolar stage may be regarded as conditioned to some extent by the radial 

 arrangement of the other elements, due in turn partly to the original epithelial 

 structure and partly, possibly, to tensions produced by the growth of the tube. 

 It is also interesting as recalling conditions in sensory epithelia and in the 

 cerebrospinal ganglia. The bipolar stage is most common probably in those 

 parts where the elements show a radial arrangement in the adult. Such are the 

 layered cortices of the mid-brain and pallium. Nerve cells maintaining a con- 

 nection, by central processes, with the luminal wall have been described in lower 

 Vertebrates. This connection may be explained as due to a persistence of the 

 central processes of cells in the bipolar stage. 



