TllE NUCLEUS 57 



instead of forming portions of the reticulum. ^^ These euchromocenters 

 appear to correspond in part to the "prochromosomes" which many 

 writers have pointed out as regions at which prophasic chromosome con- 

 densation begins. In other cases chromatic accumulations may arise 

 secondarily in the metabolic nucleus and disappear without showing any 

 direct connection with chromosome development (Kuhn, 1929a). 



The Nucleolus. — Nearly all metabolic nuclei contain one or more 

 trae nucleoli, or plasmosomes.^^ In a young nucleus just formed by divi- 

 sion there are often several small nucleoli which may unite to form two 

 or one as the nucleus becomes fully developed. In the living nucleus 

 the nucleolus appears as a dull, viscous droplet, usually round but 

 frequently irregular in shape. Centrifuging-^ and the position it natu- 

 rally assumes in certain eggs-^ show it to be heavier than the rest of the 

 nuclear matter. It may be homogeneous throughout, or it may contain 

 vacuole-like masses and occasionally small granules.-'' Chemically, 

 it is composed mainly of proteins and lipides.-^ It commonly shows an 

 affinity for acid dyes, but in some procedures it takes the basic ones. 

 Of greater interest is the fact that its chromaticity undergoes marked 

 alterations during the nuclear division cycle, as will be pointed out in later 

 chapters. Such alterations are thought to indicate interactions of some 

 sort with the reticulum, with which the nucleolus is in contact at one or 

 more points. The broad, clear zone often seen about the nucleolus in 

 fixed material is, as a general rule, an artifact due to a contraction of the 

 reticulum. 



The functions of the nucleolus are obscure, but there is obviously 

 some relation between its behavior and the cycle of alterations undergone 

 by the other nuclear constituents. This topic is to be discussed in Chap- 

 ter IX. It should, however, be pointed out here that the nucleolar 

 matter arising in the young nucleus develops principally in close associa- 

 tion with definitely localized regions of particular chromosomes, a fact 

 which has only recently come to light (p. 118). The nucleolus or nucleoli 

 tend to remain attached to such chromosomes and consequently to the 

 metabolic reticulum which the latter form. As the reticulum again 



22 Strasburger (1904c), Miyake (1905a), Rosenberg (19076, 1909ac), Overton 

 (1905, 1909), Laibach (1907), Heitz (1929a), Gregoire (1932). These are called 

 euchromocenters (true chromocenters) by Gregoire. 



23 por general accounts of the nucleolus and reviews of the literature pertaining 

 to it, see Montgomery (1899), Wager (1904), Walker and Tozer (1909), M. Jorgensen 

 (1913a), A. Meyer (1917a, 1920), Tischler (1921-1922), Ludford (1922a), and Guillier- 

 mond and Mangenot (1928). 



2* Mottier (1899), F. M. Andrews (1902), E. W. Schmidt (1914), Nemec (1929/), 

 Shimamura (1929). 



25 Gray (19276) on Echinus, Wakayama (1929) on Pinus. 



26 Digby (1910), Reed (1914), Kuwada (1919), Carleton (1920), Saguchi (19206). 

 2^ Zacharias (1885), A. Meyer (1920), Unna and Fein (1921), Fels (1926), Shinke 



and Shigenaga (1933). 



