114 The Structure oj Protoplasm 



ago, in the days when it was still commonly believed that the 

 chromosomes lose their identity at the end of mitosis and are formed 

 again in the prophase of the next division, it was noted that in some 

 organisms there are chromatic bodies in the "resting" or "energic" 

 nucleus. The chromosomes were observed to "form" in connection 

 with these, and they were therefore sometimes called prochromo- 

 somes. We cannot enter into any detailed discussion of these in 

 this short talk — for an outline of almost all that is known and of all 

 that can reasonably be guessed on the basis of present knowledge 

 reference may be made to recent publications of Geitler (1938) and 

 White (1940). In brief, the known facts from the second and third 

 levels are: (1) There are parts of the chromosomes that, in many 

 species of animals and plants, stain at all stages of the nuclear cycle. 

 (2) The sex chromosomes are particularly prone to bear such 

 "heterochromatic" regions. (3) Instead of staining at all stages, 

 they may in some species be relatively understained at times when 

 the "euchromatic" parts are deeply stained. (4) Heterochromatic 

 regions are often indiscriminate in their pairing capacity, and (5) 

 the nucleolus is very often formed in connection with one or more 

 of them. From the studies of Caspersson et al. we know (6) that 

 they are regions that differ from the rest of the chromosomes in 

 their nucleic acid cycle or total nucleic acid-producing capacity. From 

 the fourth level there may be added (7) that heterochromatic 

 regions seem to be devoid of genes that affect morphological char- 

 acteristics, and (8) that they affect the expression of genes adjacent 

 to them. 



A. characteristic that may be related to heterochromatism is the 

 tei.dency for certain regions of the chromosomes to be understained 

 after exposure to low temperatures. This has been reported in a 

 number of plants, particularly by Shmargon (1938) and other Rus- 

 sian workers, but it is particularly striking in the genera Paris and 

 Trillium in which Darlington and La Cour (1940) have studied it.^ 

 These understained "differential regions" of metaphase are, accord- 

 ing to Darlington and La Cour, the heterochromatic regions which 

 are deeply stained during the resting stage. They attribute the 

 attenuation at metaphase after cold treatment to relative "nucleic- 

 acid starvation" in these species which, they assume, normally have 

 a particularly high nucleic acid concentration. 



Schultz, Caspersson, and Aquilonius (1940) conclude that the 

 heterochromatic regions of Drosophila melanogaster have the 



' See also Wilson & Boothroyd, Cayi. J. Res. 19: 400-412, 1941. 



