FIBRILLAR SYSTEMS IN THE MITOTIC APPARATUS 163 



teins or be present in a pool. At the other extreme, we may imagine 

 the existence in the cytoplasm of preformed molecular subunits 

 which are assembled into the definitive mitotic apparatus with little 

 or no modification in their structure. The latter will be referred to 

 as the "precursor concept." It does not differ basically from views 

 held by the early cytologists, except at the level of organization of 

 the precursor units. They thought in terms of the rearrangement of 

 microscopically detectable structures present in their cytological 

 preparations, while we look upon the precursor units as being of 

 macromolecular dimensions. The problem, therefore, was to dis- 

 tinguish a net synthesis, on the one hand, from the spatial rearrange- 

 ment of pre-existing structures, on the other. The advent of the 

 technique for obtaining isolated mitotic apparatus suggested an ex- 

 perimental design which could decide between these two extremes. 

 Immunology offered potentially a suitable method for attempting 

 to identify some component of the unfertilized egg with a structural 

 component of isolated mitotic apparatus. If the mitotic apparatus 

 were formed de novo, it would be reasonable to expect the complete 

 absence of its structural constituents from the unfertilized egg. The 

 precursor concept, as stated earlier, would predict the existence in 

 the mitotic apparatus of molecules identical to a species present in 

 the unfertilized egg. 



The antigen-antibody reaction is one of the most specific chemi- 

 cal reactions known ( Landsteiner, 1945), but this in itself did not 

 provide a suitable means to identify with each other the same molec- 

 ular species from different sources. An added complication was the 

 expected heterogeneous nature of the reactants. The gel-diffusion 

 technique, as originated by Oudin ( 1946 ) and modified by Ouch- 

 terlony ( 1949), provided the ideal solution to these dilemmas. Two- 

 dimensional gel-diffusion techniques (Oudin, 1952; Ouchterlony, 

 1958) can resolve very complex antigen-antibody reaction systems 

 into a series of discrete bands of precipitate immobilized by the gel 

 medium, and further provide an extremely sensitive means for es- 

 tablishing an identity or non-identity between antigens from differ- 

 ent sources. When bands of precipitate assignable to different 

 antigen solutions fuse or merge, instead of intersecting, the respon- 

 sible antigens are considered to be immunologically identical. 



What restrictions must be imposed upon an identity between two 

 proteins as established by the fusion of two bands in gel-diffusion 

 analysis? It seems reasonable to expect that identical antigenic 



