230 



FINE-STRUCTURE OF PROTOPLASM II 



as there is not the slightest indication of the existence of a chain lattice. 

 All the same, the anisotropy of swelling and the cleavabiHty of the 

 chromosomes are in favour of an orientation along the long axis of 

 the protein ground mass. In his microchemical experiments with 

 chromosomes of salivary glands Painter (1941) is also impressed by 

 their fibrillar character. It must be supposed that the isotropy of the 

 protein results from coiling and folding of polypeptide chains, com- 

 bined with a corresponding hydration. Evidently the chromosome 

 protein consists rather of globular molecules which may be aggregated 

 to form beaded chains. 



The explanation of the birefringence of other chromosomes 

 (Schmidt, 1957a, 1941a) is much impeded by our insufficient 

 knowledge of the submicroscopic orientation in the chromonemata 

 bundles of the salivary glands. According to Becker and Kozbial 

 (1937), the optical character of the chromosomes of the root tips of 

 Allium and Vicia depends on the process of fixing : if treated directly 

 with alcohol they appear to be negative; after a previous treatment 

 with acetic acid vapour (causing swelling) they are positive. On the 

 assumption of a nearly complete scattering of the nucleic acid chains 

 in the isotropic living chromosome, these effects might be explained 

 by tendencies towards orientation as a result of the shrinkage or 

 swelling in the fixing process. It seems to me that considerations of 

 this kind open more prospects than explanations formerly attempted 

 with the aid of the spiral structure (Nakamura, 1937). Kuwada and 

 Nakamura (1934) explain the positive double refraction of the chro- 

 mosomes of Tradescantia by a single spiral of negative chromonemata ; 

 whereas, in their opinion, optically negative chromosomes are caused 

 by a double- wound spiral. 



d. Submicroscopic Morpholo^ of Hereditary Processes 



Genes. The fibrillar character of the chromatids meets two important 

 morphological requirements of genetics: i. the substrate is easilj 

 ckavable in the direction of the long axis, which is not only necessary for 

 the splitting of chromosomes but also for the phenomena taking place 

 between synapsis and diakinesis in heterotypic divisions; 2. the long 

 chromonemata offer an opportunity for the linear arrangement and the 

 possibility of exchange of the genes. 



AIorgan's school has calculated that the number of genes known 



