Z NUCLEUS 229 



scopic fine-structure of the chromomeres Schmidt therefore takes 

 into consideration a possible arrangement as given in Fig. 125 c. The 

 fact, ascertained by Astbury and Bell (1958), that the fibre period 

 of 3.34 A of the nucleic acids is about the same as that in the poly- 

 peptide chains (3.5 A, see Table XXXII, p. 368) seems to support 

 this hypothesis. 



With the aid of the ultraviolet dichroism of the nucleic acid chains 

 Caspersson (1940b) has checked the structure proposed in Fig. 125 c. 

 If the nucleic acid molecules in the protein fibres showed complete 

 orientation, chromomeres, like artificial thymonucleic acid fibres, 

 would displav a very pronounced dichroism in polarized ultraviolet 

 light. Compared with these fibres, however, the chromomeres of the 

 chromosomes of the salivary glands show only an extremely small 

 dichroitic effect. Caspersson therefore draws the conclusion that the 

 nucleic acid chains are intercalated practically without orientation. 

 Also the double refraction of the chromomeres, as derived from the 

 birefringence of flow of sodium nucleate sols (Signer, Caspersson 

 and Hammarsten, 1938), proves to be very small. Meanwhile, as- 

 suming that nucleic acids are straight chains, the negative sign of the 

 chromomere birefringence indicates that the chains have a certain 

 preferred orientation. With the aid of the formula mentioned on p. 90 

 the scattering in the orientation of the chain molecules can be calcu- 

 lated (Frey-Wyssling, 1943b), and the scattering angle found in this 

 way is 86°. 5, i.e., nearly a right angle. This means that the scattering 

 is almost complete, thus furnishing an important argument against 

 the supposition that the nucleic acid molecules are parallel to the 

 chromonema axis. A similar result is obtained if the intrinsic double 

 refraction of — 0.050 found by Schmidt (1928) for the chromatin of 

 the Sepia sperm, or even only a fraction of this value, is compared 

 with the birefringence of the chromomeres. 



In spite of the small orientation of the nucleic acids, Caspersson 

 assumes the protein chain structure to be continuous. Orientated 

 polypeptide chains are supposed to cause the anucleal chromosome 

 segments to appear positively birefringent. This effect, however, can 

 only be observed in stretched chromosomes (F. O. Schmitt, 1938; 

 Pfeiffer, 1 941 a). Optics therefore do not provide sufficiently reliable 

 data to assume orientation of the protein chains. There can certainly 

 be no pronounced fibrillar texture of expanded polypeptide chains. 



