THE NUCLEUS 65 



"chromatin" refers to the basichromatin. Oxychromatin appears to be 

 closely similar in composition to the achromatic structures in the cyto- 

 plasm, such as spindle fibers and centrosomes. The prominent place 

 occupied by the nucleus in cytology is due in large measure to the con- 

 spicuous behavior of its chromatic substance at the time of cell-division 

 and fertilization, topics which are to receive detailed consideration in 

 subsequent chapters. 



In many nuclei basichromatin accumulates at certain points in the 

 reticulum, forming karyosomes, also called "net knots" and chromatin 

 nucleoli. These seem to be masses of surplus chromatin elaborated by 

 the nucleus during the resting phase or in some cases chromatin which 

 has flowed to these points from the other parts of the reticulum. During 

 the next mitosis they are distributed with the rest of the chromatin. As 

 Rosen (1892) long ago showed by his studies of their staining reactions, 

 they differ decidedly in composition from true nucleoli, although they 

 may closely resemble the latter after treatment with certain stains (iron- 

 alum-hsematoxylin) . 



One or more true nucleoli, or plasmosomes (Ogata 1883), are usually 

 present in the nucleus. A single nucleolus is probably characteristic of 

 most nuclei ; there are rarely many, and in some cases there is none. The 

 nucleolus may be in close organic connection with the nuclear reticulum 

 or it may lie entirely apart from it. In composition it consists largely of 

 such oxychromatic substances as plastin and pyrenin, or of nuclein well 

 saturated with protein (Zacharias). It usually stains with the acid 

 dyes: by a proper selection of stains it may, therefore, be distinguished 

 from the karyosomes, which, being composed of basichromatin, take the 

 basic dyes as a general rule. In structure the nucleolus may appear to 

 be homogeneous throughout, like an oil globule; in other cases it has an 

 outer envelope of different consistency and staining reaction. Very often 

 vacuoles, occasionally containing granules, are present in the interior. 

 Crystalloid bodies are also frequently observed in the nucleolus (Digby 

 on Galtonia, 1910; Reed on Allium, 1914; Kuwada on Zea, 1919). In 

 the epithelial cells of the frog intestine Carleton (1920) finds one or more 

 intranucleolar bodies which he calls "nucleolini." These appear to 

 divide and pass to the daughter cells at the time of mitosis, and may 

 possibly initiate the formation of new nucleoli in the daughter nuclei. 

 Montgomery (1899) 1 concluded that the nucleolus grows in size by the 

 apposition of smaller particles of nucleolar material on its surface, and 

 by the intussusception of vacuolar substance arising outside the nucleolus. 



Function of Nucleolus. Various opinions have been entertained re- 

 garding the function of the nucleolus. By many workers it has been 

 looked upon as chiefly a passive by-product of no further use in the life 



1 An exhaustive review of the literature dealing with the nucleolus up to 1899 

 is given in this paper. 

 5 



