56 



INTRODUCTION TO CYTOLOGY 



or a looser combination of two or more constituents, only one of which is 

 "chromatin" (in the narrow sense). 



In nuclei with more sharply differentiated chromatic lumps and 

 achromatic connecting strands the conception of two substances in the 

 reticulum may seem more attractive. Studies on the chromosomal 

 threads (chromonemata) during nuclear division have shown that the 

 lumps {chromomeres) may be located in definite positions in the threads, 



which in such a case have characteristic structural 

 patterns (Fig. 79). Certain of the large lumps have 

 been identified in the reticulum, and it is inferred 

 that the small ones may also persist in some measure. 

 Hence such chromomeres can scarcely be chance 

 accumulations of chromatic matter; they are definite 

 differentiations which there is much reason to believe 

 are associated in some way with certain functional 

 activities. They might be regarded as local swellings 

 of the karyotin (one-substance theory) or as regions 

 where "chromatin" is produced in characteristic 

 amounts (two-substance theory). It may be that 

 we shall come to look upon "chromatin" not as a 

 substance existing primarily as definite units but 

 rather as one whose presence indicates activities in 



^^^ 



I 



Fig. 



B 



24.- 



Kuhn, 1929a.) B, nu- 

 cleus of Impatiens in 

 prophase, showing eu- 

 chromocenters at spin- 

 dle-attachment regions 

 of developing chromo- 

 somes. (After Gregoire, 

 1932.) 



cieus of Phaseoius with other localized units or regions in the nuclear threads. 



chromocenters. (After 



The above uncertainties do not prevent us from 

 carrying forward the conception of the reticulum as 

 a system of chromosomal threads connected by fine 

 strands, or from speaking of the reticular substance 

 in general as karyotin. As will be made evident 

 in later chapters, the nucleus may be regarded as a 

 group of closely associated chromosomes performing their metabolic 

 functions together. 



In many nuclei there are at certain stages one or more conspicuous 

 masses of karyotin at several points in the reticulum (Fig. 24). Of 

 the many terms applied to these the most suitable seems to be chromo- 

 centers.^^ After certain stains (iron alum-hiematoxylin) they may at 

 times closely resemble true nucleoli, but the distinctness of the two was 

 long ago demonstrated with other stains (Rosen, 1892). In some 

 cases it has been shown that they represent definite chromosomal regions 

 which remain condensed and highly chromatic ("heteropyknotic") 



21 Baccarini (1908). They have been variously known as net knots (Flemming, 

 1882), karyosonies (Ogata, 1883), pseudonucleoli (Rosen, 1892), Nebennucleoli (Zacha- 

 rias, 1895), and chromatin nucleoli. Karyosome is a widely adopted term, but the 

 fact that it has been applied to other quite different elements has made the substitu- 

 tion of another term advisable. 



