FIBRILLAR SYSTEMS IN THE MITOTIC APPARATUS 169 



solve only the fibrous components of the mitotic figure, leaving 

 intact the centrosomes and chromosomes (Mazia, 1958). These 

 are not the conditions usually considered appropriate for reducing 

 disulfide bonds. Yet the success of isolating the mitotic figure from 

 living material depends upon treating it as though it were held 

 together by disulfide bridges (Mazia, 1959). The apparent incon- 

 sistency between the ease with which the isolated mitotic apparatus 

 may be dissolved and the need, during the isolation procedure itself, 

 to use an agent ( dithiodiglycol ) that presumably stabilizes disulfide 

 links, remains to be resolved. There is, thus, no clear-cut experi- 

 mental evidence that directly corroborates the involvement of inter- 

 molecular disulfide bridges in the structural organization of the 

 mitotic apparatus, nor do we have convincing proof that sulfhydryl- 

 disulfide interchanges play an important role in the functional 

 activity of the mitotic apparatus. However, we have enough data to 

 say that protein molecules containing reactive sulfur groups par- 

 ticipate in some manner in the structure of the mitotic figure, and 

 we can only speculate about the manner in which the sulfur groups 

 react with each other to confer strict spatial orientation to the mole- 

 cules bearing them. The possible mechanisms of sulfhydryl-disulfide 

 interchanges and how these may be invoked to bring about the 

 polymerization of the precursor molecules into the mitotic figures 

 are adequately discussed by Jensen ( 1959 ) and Mazia ( 1959 ) . 



Immunochemical results allow us to consider the formation of 

 the mitotic apparatus in terms of the aggregation of precursor sub- 

 unit molecules that are present in the unfertilized egg, and the 

 endeavors of other investigators provide indirect evidence that pro- 

 teins bearing reactive sulfur groups perform important roles in the 

 orientation of the precursor molecules in the mitotic apparatus. 

 Gross ( 1954 ) observed that extracts of Arhacia eggs contained cal- 

 cium-insoluble protein and speculated on its importance to sol-gel 

 transformations. A similar protein was subsequently detected in the 

 soluble fraction of a O.IM KCl extract of unfertilized S. purpuratus 

 eggs. This protein appeared as a clear fibrous precipitate when 

 CaCL to a final concentration of 0.05M was added to the unferti- 

 lized egg extract. It could be redissolved by dialysis against distilled 

 water. Some of the physical properties of this protein, which has 

 beerf termed the calcium-insoluble fraction, were studied by Kane 

 and Hersh ( 1959). Their results indicated a strong resemblance be- 

 tween this protein and the protein present in solutions of dissolved 



