60 Nature of the Genetic Material 



Unfortunately, there is no chemical definition. Some hetero- 

 chromatin is clearly very rich in DNA; in other cases it is reported to 

 be composed mainly of RNA. The facts are so contradictory that it is 

 not worth while to quote them. Actually, it seems probable that 

 heterochromatin is the seat of chemical processes which change its 

 composition constantly, processes which most cytologists assume to be 

 connected with protein synthesis, apart from genie protein. In favor 

 of this is the fact that the nucleolus is frequently in a heterochromatic 

 section. Heterochromatin is so elusive a structure that it would be 

 better not to attempt a theoretical discussion, were it not that we 

 know a number of interesting features which in the cells where they 

 are found are perfectly clear and straightforward. Therefore it seems 

 advisable — we have already called it the pragmatic attitude — not to 

 dwell upon what we do not know of heterochromatin but to pass in 

 review the available positive and invariable facts. There are purely 

 cytological facts as well as cytogenetic features around which a fruit- 

 ful discussion can be built; these show that heterochromatin, whether 

 we can define it or not, must have important functions in the cell. 



The most conspicuous group of facts is the one relating to entire 

 heterochromatic chromosomes and their fate. These facts stand some- 

 what apart from those generally found, in which heterochromatic 

 sections of the chromosomes show the different types of allocycly. 

 The completely heterochromatic chromosome may be considered the 

 extreme end of a series of conditions which are more widely spread 

 but do not give such clear information. In detail we refer to three 

 somewhat different types : ( 1 ) one in which heterochromatic chro- 

 mosomes are preserved only in the cells of the germ track; ( 2 ) one in 

 which heterochromatin is not organized into chromosomes and re- 

 served for a special type of cells; and (3) a type involving the 

 existence of heterochromatic chromosomes which are not complete 

 chromosomes, the so-called supernumeraries, but, nevertheless, show 

 some preferential distribution upon specialized cells. 



b. Chromatin diminution and related phenomena 



The first group contains the different cases of so-called chromatin 

 diminution. The classic case is that of Ascaris (Boveri), now called 

 Parascaris equorum. It is a little complicated by the fact that the 

 chromosomes in the sex cells are collective chromosomes joined to 

 each other end by end and having many centromeres and spindle 

 fibers. This condition is not easily explained, but it has been found that 

 frequently chromosomes tend to such more or less irregular associ- 



