THE NERVOUS SYSTEM. 485 



The next stage is a monopolar stage produced by the atrophy of the luminal 

 process. Cells in this stage are the neuroblasts of His, the peripheral processes 

 being the developing axones (Fig. 425). As seen in ordinary stains, the above 

 differentiation of the neuroblasts is marked by a corresponding differentiation 

 of the nuclear layer into an inner layer retaining its previous characteristic radial 

 arrangement, and an outer layer characterized by fewer nuclei more irregularly 

 arranged. The latter layer is the mantle, or neurone layer (Fig. 442) . There 

 are now three layers: (i) inner (nuclear), (2) mantle (neurone) and (3) marginal. 

 The mantle layer is thus produced by the migration and differentiation of cells 

 into neuroblasts. While this process may begin near the lumen (apolar nerve 



FIG. 425. Dorsal portion of the lumbar cord of a chick embryo of three days. Cajal. 

 A, B, Cells in the apolar stage with fibrillogenous zones; B shows transition to the bipolar stage; 

 E, further advanced bipolar cell; G, cells in monopolar stage or neuroblasts of His; a, giant 

 cone of growth. These cells have migrated to the outer part of the nuclear layer, thereby 

 forming the beginning of the mantle layer. 



cell of Cajal) and progress as the cell has moved somewhat further away (bipolar 

 stage), the monopolar stage is probably reached only when such cells form a part 

 of the mantle layer. In other words, the mantle layer is created by the migra- 

 tion to a certain location and differentiation to a certain stage of the primitive 

 nerve cells. The mantle layer, as previously stated, probably also contains 

 indifferent cells which may by further proliferation and subsequent differentia- 

 tion become either glia or nerve cells.* The looser arrangement of the cells of the 

 mantle layer is probably in some measure due to the growth of the dendrites which 

 appear soon after the axones. It may be also due to the beginning vascularization 

 of the tissues with resulting transudates (His) which usually, however, begins 

 somewhat later. The association in time of vascularization and further growth 



* It is an open question as to how late in development these " extra ventricular " cell- divisions, in- 

 volving " indifferent " cells, may occur. The neuroglia cells, however, like other supporting elements, 

 preserve this capacity of division indefinitely, as shown by the increase in neuroglia cells in patho- 

 logical conditions. 



