Genie and Non-genic Parts of the Chromosome 71 



favorable material, a rather large number of important facts are avail- 

 able for theoretical analysis. I shall arrange these facts according to 

 the way I interpret their interrelations. I consider as the first decisive 

 fact, both for cytological description and genetic interpretation, the 

 distinction between chromocentral and intercalar}' heterochromatin. 

 In a general way this means heterochromatin all of which enters into 

 the framework of the chromosomes during mitosis, but which during 

 interphase as well as meiotic prophase, and also in nuclei that have 

 ceased to divide, shows its two different t\pes. The first t>"pe, chro- 

 mocentral heterochromatin, is removed from its place in the chromo- 

 somes and combined in a single chromatic mass, the chromocenter. It 

 is contained in the mitotic chromosomes in large blocks and may even 

 constitute an entire chromosome, as in the Y-chromosome in D. 

 melanogaster (and also the cases mentioned in the preceding chapter). 

 The other t\"pe of heterochromatin is found in smaller sections over 

 the length of the individual chromosomes. This intercalar\' hetero- 

 chromatin is structurally and quantitatively different from the chro- 

 mocentral t>"pe and, in addition, shows different cNtological affinities 

 and genetic significance. We would be completely justified in restrict- 

 ing the name 'lieterochromatin" to the chromocentral t\pe and in 

 using a special term for the intercalar>'. 



In the discussions of the preceding chapter this distinction was 

 hot made. In Ascaris we dealt with chromocentral heterochromatin, 

 in some of the discussions (e.g., Serra's theory- ) mostly N^ith inter- 

 calary-. The following analysis will shed hght also on the former 

 material. 



aa. Chromocentric and Y-chromosomal heterochromatin 



The chromocentric heterochromatin of D. m-elano garter consists 

 of three parts (see fig. 9): (1) rather large blocks surrounding the 

 centromere in the second and third chromosomes, probably also in 

 the fourth; (2) one large block at the spindle fiber end of the 

 X-chromosome, containing the nucleolus-organizing region; (3) the 

 entire Y-chromosome, also \\-ith a nucleolar organizer. -\11 these parts 

 were distinguished by Heitz ( 193.3 ) in the mitotic metaphase by 

 distinctive staining. They show their nature in many interphase nuclei 

 by combining in a chromocenter (fig. 10). This is best known from 

 the salivary gland nuclei, in which all these sections, including the 

 entire Y, are united in a chromocenter to which the chromosomal loops 

 are attached. The saHvar\- gland chromosomes are indicated in figure 

 9; the lines showing identical points in mitotic and salivar>- chromo- 



