THE STRUCTURAL COMPONENTS OF PROTOPLASTS 



27 



In some nuclei there are several dense, highly stainable lumps in the 

 midst of the chromonemata (Fig. 16). These are chromocenters and seem 

 to represent regions where the chromonemata are more closely packed and 

 have retained more of the matrix. Such regions occur in definite positions 

 in certain chromosomes. When each chromosome has a single one, the 

 number of chromosomes can sometimes be determined by coimting the 

 deeply stained spots in the metabolic nucleus. Ordinarily chromosomes 

 can be counted only during nuclear division. 



Each typical nucleus has one or more nucleoli lying in the karyolymph. 

 These differ chemically from the material of the chromocenters and can 



^i^.:':-m 



^-iS- • •.♦ 



-.%:'-' 



Fig. 15. — Living nucleus in Tradescantia 

 .stamen hair mounted in paraffin oil. The 

 mottled appearance is due to the numerous 

 fontorted cliromonemata. (Photograph hy 

 H. Telezynski.) 



B 

 Fig. 16. — A, nucleus of bean (Phase- 

 olus), with chromocenters. {After E. 

 Kuhn.) B, cell of touch-me-not (Impa- 

 tiens), with nucleus in prophase. Eu- 

 chromocentera are present (see page 87). 

 (After V. Gregoire.) 



easily be distinguished from them by suitable staining methods. They 

 develop in close union with certain chromosomes in the newly formed 

 nuclei at the close of division; this is why they usually lie against one or 

 more of the chromonemata in the metabolic nucleus. Their number also 

 is related to their connection with chromosomes. These points will be 

 discussed in Chap. YH. 



The physical consistency of the nucleus as a whole obviously depends 

 upon the consistencies and relative amounts of its several components — 

 membrane, karyolymph, and chromonemata. In certain animal eggs it 

 behaves under the mici'omanipulator like a very fluid droplet with a 

 firmer membrane. In cells of several other kinds it seems to be more 

 viscous throughout than the cytoplasm and can be moved about bodily 



