Thc method to he described in this report is ilie outcome of ;~n attcmpt to find a simple and i*eliablc techiiique for the demonstration of rriyelinatetl arid unmycllinatecl ncrve fibers in parafin series of the central nervous system of riormal and experirnental aiiirrials. It is ;i well-lrnown fact that the use of r c d a c d silver methods in cxperirnentnl investigations of the h e r connect ions of tho central iier\wis system has been limited either by t h ( 2 uuccrtainty of results obtaimblc or hy the complexity of the methods. Complex methods are so firnecaonsuming as to makc. tliclir use almost impossible in the study of serial sections of l h e brains of larger animals. It is most unfortunatc that this is tlic case, because the reduced silver nieihods, if properly cwnt,rollrd, are siiperior in inany respects to both the Xlarchi and Weigcrt niethoc-ls for (~xperirriental work on fiBw wnnections, especially sirice neither of the latter mclthocis is capable of sbowing changes i n unmyelinated fibers or in nerve endings.It is uiinecessary to discuss at leiig-th the many advantages of the silver.-iriiyiregnation methods for paraffiii sertions over the ~)Iock-irnpre~iiatio~i methods. E:spc?cially important are the possibilities of more imi form staiaing, of staining ultcmatc sections with silvcr and with other rriethods, such as 'This lsork wm aided by i l g m i t t o The Vnivcrsity of Chicago by the Rockrf e l l t~ Foondation. 89

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The layers and cells of the early developing central nervous system lack direct counterparts in the adult and must be designated by a special terminology. The inconsistent and inaccurate langua,ze now in use leads to misunderstanding and a revision is proposed in which the four fundamental zones are termed the ventricular, subventricular, intemediate, and marginal zones. Each is defined according to the form, behavior, and fate of its constituent cells. All neurons and macroglia of the central nervous system can be derived from these developmental zones.