20 Nature of the Genetic Material 



meres; and even none, if the chromomeres are not an accumulation of 

 DNA but chromonema whorls, which is not probable. Only one im- 

 portant diflFerence is visible: wherever spontaneous or experimental 

 breaks of the chromosomes can be seen microscopically (in salivary 

 chromosomes and some pachytene chromosomes) they are located 

 between the chromomeres. This shows that the processes leading to a 

 break and to reunion of broken ends are confined (or at least more 

 easily accomplished ) between the chromomeres, or perhaps are visible 

 only between the chromomeres. (Actually we have reason to believe 

 that breaks within single salivary bands may occur, though the micro- 

 scope does not reveal them.) The visible occurrence of breaks 

 between the chromomeres does not necessarily mean that the chro- 

 monema has here a different function. It might be a purely mechanical 

 cause which keeps chromonema incrusted with much DNA or coiled 

 into a tight whorl (in the chromomeres) from being easily broken. 

 We have very little information on the visible relation of chromo- 

 nema and chromomere. Salivary gland chromosomes after treatment 

 with alkali change their structure so as to reveal within the chromo- 

 mere what looks like a coil of the chromonema to which droplets of 

 chromatin are attached (Kodani, 1942; Goldschmidt and Kodani, 

 1942). However, it is very difficult to say what is normal and what is 

 an artifact in this remarkable aspect. Bauer's (1952^) statement that it 

 is simply an artifact is categorical but difficult to prove. He forgets 

 that an experiment consists in production of a controlled artifact from 

 which to draw conclusions about the normal condition. In the present 

 case the "artifact" is a typical, orderly structure, always produced 

 identically with the same treatment. This must somehow be based 

 upon a definite structural condition, which has to be analyzed. 



It is possible to interpret the characteristic structure of the lamp- 

 brush chromosomes in the vertebrate oocyte similarly. Attached to the 

 chromomeres is a ring or rosette of fine loops of the thin chromonema 

 (which accounts for the name "lampbrush" chromosomes; Riickert, 

 1892). If they were attached to a fine chromonemal coil (which can- 

 not be seen) and incrusted with DNA (which in this stage is not the 

 case), the same structure as in treated salivary chromosomes would 

 be present. (See Duryee, 1941 ff.; Dodson, 1948; Guyenot et al., 1950; 

 Callan, 1952; Gall, 1954; Alfert, 1954.) Ris (1945) maintains that the 

 chromomeres are loops of the chromonema. Thus it is rather probable 

 that the chromonema, containing only small amounts of nucleic acid, 

 is structurally and chemically continuous, but makes periodically tight 



