Genie and Non-genic Parts of the Chromosome 23 



monema in equal amounts and that the chromomere is therefore a 

 kind of knot in the chromonema (loop, according to Ris, 1945), which 

 appears more "chromatic" because of the many layers of DNA in a 

 whorl-like structure. I still believe that the very regular artifacts 

 produced by Kodani (1947) in salivary chromosomes are indicative 

 of a real structure. A decision is diflBcult in view of the fact that one 

 author finds only four strands in the Drosophila salivary chromosomes, 

 another sixteen, and still another one thousand! AU these data and 

 claims must be brought into line if we are to derive insight into the 

 genie properties of the visible structures from morphological work. 



The synaptic attraction chromomere by chromomere is one of the 

 major riddles of cytology. Many geneticists and biophysicists ( Muller, 

 1947; Friedrich-Freksa, 1940; Jehle, 1952; Delbriick, 1941) have pro- 

 posed explanations for the attractive forces, which seem to be beyond 

 a physical explanation. For our present discussion we ask only whether 

 a solution of the physical problem helps our understanding of which 

 part of the chromosome is endowed with genie properties. Certainly 

 synapsis is one of the basic features of genetics, without which Men- 

 dehan inheritance and crossing over would not be possible. There is 

 no indication thus far whether the genetic material has anything to do 

 with the phenomenon, whether it occurs on the all-chromosomal level 

 or whether only the centromeres or other parts of the chromosome are 

 decisive. Though the microscope reveals a synapsis chromomere by 

 chromomere, the ignorance of what a chromomere is makes it im- 

 possible to decide whether the attraction is only between two ex- 

 tremely folded stretches of a macromolecule or anything else, and, 

 further, why it is present or absent in the difiFerent stages of the 

 chromosomal cycle but partly present in somatic cells of Diptera. No 

 primary problem of cytogenetics is more obscure than the processes at 

 synapsis and crossing over. The brilliant theory of Darlington ( 1937 ) 

 amounts, at close sight, to a restatement of the observed facts in terms 

 of unknown forces. Even the purely morphological processes at the 

 time of crossing over (including the time itself) are unknown and the 

 subject of dissension. Thus a more detailed discussion would hardly 

 help our present understanding of the genie material in the chromo- 

 some. But some remarkable trends may be pointed out. 



Practically all geneticists are convinced that crossing over involves 

 breakage of chromatids and reunion. There are powerful facts in 

 favor of this, apart from the cytological details, which still, after all 

 the work of devoted specialists, permit such difiEerent explanations as 



