PHYSICO-CHEMICAL ASPECTS I3 



fibres and chromosomes etc. In the cases of the chromo- 

 some fibre, muscle fibre and collagen fibre it is known 

 that there is a good deal of differentiation along the 

 length of the chains. It is likely that this is also the case 

 with most other natural fibres. Except in the case of the 

 chromosome, it is not at all clear what significance 

 should be attached to the local differentiation along the 

 length of the fibre. One theory is that it assists in the 

 precise adlineation of polypeptide chains, thereby giving 

 rise to fibres of maximum strength. 



A physically quite different type of structure, also 

 composed of adlineated protein molecules, is the type 

 found in tobacco mosaic virus tactoids. These consist of 

 needle-shaped molecules, which are oriented parallel to 

 one another, but which are not in contact at any point. 

 They are maintained in these positions, with the distance 

 between the long axes of the molecules relatively well 

 defined, by long range forces whose nature is not yet 

 clearly understood. Bernal has supposed that the 

 spindle and asters of a dividing cell are composed of 

 such tactoids. It is also possible that the forces acting 

 between daughter chromosomes during mitosis are of 

 this type, that cell adhesions and the form of cells are 

 in part determined by such forces, and even that the 

 differentiation of cells is influenced by forces of this type. 

 In addition to the gels or tactoids mentioned above, 

 there are the cortical gels which do not seem to consist 

 of very highly oriented molecules. They may have a 



