2 NUCLEUS 223 



elements of the spindle must be amicroscopic, consisting, maybe, of 

 polypeptide chains. Whether these are individualized or in some way 

 aggregated cannot be decided. In any event they must be arranged 

 in orderly array to produce the observed birefringence. As a result of 

 the fixing process, the visible fibres arise by dehydration and by some 

 kind of crystalline aggregation of these birefringent elements. Their 

 denser packing causes a marked increase in optical anisotropy. As the 

 birefringence is positive, the assumption of expanded polypeptide 

 main valency chains is not unreasonable. At any rate this hypothesis 

 may be used as long as it is not disproved. 



The spindles are formed primarily in the cytoplasm when the 

 nucleus is still intact. In some cases even cells devoid of nuclei are 

 capable of forming spindles (E. B. Harvey, 1936). Having supposed 

 the cytoplasm to consist of proteins, there is no difficulty in deriving 

 the spindle structure from the cytoplasmic frame structure. The 

 globular macromolecules must simply denature to give expanded 

 chains which aggregate laterally. In fixed preparations this fibrillation 

 of the cytoplasm can often be observed in the regions surrounding 

 the poles. I fully realize that the transition will not take place according 

 to this simple scheme, but must be connected with the synthesis of 

 additional protein chains. However, the principal condition is that the 

 cytoplasm already contains the structural elements required, i.e., the 

 polypeptide chains, either as structural material or as a model for the 

 formation of new chains. 



The spindle is not always formed outside the nucleus; in special 

 cases it has its origin inside the nuclear boundary, or it is observed 

 that cytoplasmatic and nuclear fibrils together take part in the con- 

 struction of the spindle. This once more indicates that the submicro- 

 scppic structures of cytoplasm and nucleus are alike. By submicro- 

 scopic changes fibrillar elements of similar morphological nature may 

 originate from both of them. We may conclude that the nucleus does 

 not separate chemically from the cytoplasm as a completely foreign 

 substance. 



The protein chain structure of the spindle fibres can be utilized in 

 the so-called "strain theory" (Zugfasertheorie), according to which 

 the chromosomes are drawn towards the pole by shortening fibres. 

 In fact, expanded polypeptide chains have the property of contracting 

 considerably under certain circumstances (see p. 134). It might be 



