H. E. LEHMAN 207 



and Shengiin and of Beermann indicates, in Chironomus larvae 

 at least, that in addition to other more easily recognized nuclear 

 variations there are also clear-cut tissue-specific differences in 

 the fine structure of chromosomes. These authors consider the 

 possibility that these differences in structure and staining proper- 

 ties of comparable bands on homologous chromosomes may be 

 evidence of activity or inactivity of different specific genes in the 

 respective tissues. A similar suggestion has been made in connec- 

 tion with the possible functional significance of the "lamp brush" 

 chromosomal structure of amphibian oocyte nuclei; the localized 

 enlargement of certain loci on the chromosomes may indicate 

 multiplication of specific regions of the chromosome associated 

 with special syntheses in the developing germ cell (see Duryee, 

 1950; Gall, 1952). 



A final category of cytological evidence for nuclear differentia- 

 tion is associated with variability in heterochromatic material 

 demonstrable in interphase nuclei at a time when the nucleus is 

 physiologically most active (Berrill and Huskins, 1936; Huskins 

 and Steinitz, 1948a ) . In a number of instances interphase hetero- 

 chromatin can be traced to specific chromosomes during mitosis 

 and has been useful in determining ploidy in interphase nuclei 

 (Geitler, 1934, 1941; Huskins and Steinitz, 1948b). Heterochro- 

 matin is considered to be for the most part genetically inert (as, 

 for example, in the X and Y chromosomes of Drosophila and in 

 the eliminated chromatin in somatic cells of Ascaris and Sciara); 

 nevertheless, duplications, deletions, and translocations of hetero- 

 chromatin are known to produce sterility and a variety of "posi- 

 tion effects." There has been much speculation concerning the 

 possibility that variations in heterochromatic content might be 

 evidence of differential gene action as was suggested by Muller 

 and Gershenson (1935; see also Schultz, 1939, 1947, 1952; 

 Brachet, 1947; Huskins, 1947; Lewis, 1950; Hannah, 1951; Cas- 

 persson, 1950; Caspersson and Schultz, 1951; Gall, 1952). As an 

 example of one of the present trends of thought, Schultz ( 1952, 

 p. 38) suggested that only the specific genes required for function 

 in a particular type of cell are in an active state; the others are 

 in the "heterochromatic" state and could be dispensed with with- 



