NUCLEUS 



227 



chromomeres. With the aid of digestion experiments in which the 

 nucleic acid was protected from digestion by lanthanum thymo- 

 nucleate, Caspersson (1936) finds that the chromomeres are resolved 

 into extremely thin discs. Ultraviolet photography reveals a fine- 

 structure of lamellae with a thickness 

 of only 0.1 j.1. Since at this order of 

 magnitude the limit of the resolving 

 power in ultraviolet hght is reached, 

 the question as to whether these very 

 thin chromomere discs possess a still 

 finer submicroscopical structure and 

 thus are subdivided remains unsettled. 

 Personally I do not doubt that they are. 



Conversely, milt nuclease digests 

 the nucleic acids of the chromomeres 

 (Mazia and Jaeger, 1939) without 

 disturbing the ground structure of the 

 chromosomes of the salivary glands. 

 The ability to take the Feulgen stain 

 disappears; on the other hand the 

 ninhydrin test turns out positive over 

 the entire length of the chromosome. 

 So the chromonema does not consist 

 of alternating protein and nucleic acid 

 links, but represents a continuous 

 protein thread in which at regular 

 intervals nucleic acid knots are inter- 

 calated. The nucleic acids form saltlike 

 compounds with the protein ground 

 mass, the nucleoproteins, whose oc- 

 currence is therefore limited to the 

 chromomeres (Fig. 125 b-d). 



Fibrillar hypothesis. From a morphological point of view Wrinch 

 (1936) believes the molecular structure of the chromonema to be as 

 follows : the polypeptide chains form a system of parallel fibrils like 

 the warp of a weaving-loom and the nucleic acids represents the woof 

 in this system of chains. Every four neighbouring polypeptide chains 

 are kept together by a molecule of the tetra-basic thymonucleic acid. 



Fig. 124. Two incompletely conjug- 

 ated giant chromosomes of the nuclei 

 from the salivary gland of a Drosophila 

 hybrid with a chromosome pattern 

 characteristic of the two parental 

 species (from Patau, 1935); mel from 

 Dr. melaiiogasier, sim from Dr. simu- 

 lans ; in a a. structural difference. 



