180 



STRUCTURE OF THE 'NUCLEUS. 



The chromoplasm usually takes the form of a network of fibres connected 

 externally with the nuclear wall. One or more strongly refracting granules or 

 globules, which are named nucleoli, may be suspended, isolated, in the nuclear 

 matrix, or they may be connected with one another, if they are more than one, and 

 with the nuclear wall by the network of fine filaments just spoken of (fig. 206). Some 

 authors regard the nucleoli as mere local accumulations of the chromoplasm, 



Fig. 206. DIAGRAM OF RESTING NUCLEUS. (Waldeyer.) 



Fig. 207. CELL-NUCLEI, EXHIBITING THE RETICULATE APPEARANCE OF THE KARYOPLASM. (Flemming.) 

 a, nucleus of liver-cell (carp) ; b, nucleus of a connective tissue cell. 



differing only in size from those which occur at the nodes of the network. Others, 

 relying upon certain differences in their behaviour to staining fluids, look upon the 

 nucleoli as altogether distinct in nature and chemical composition. 



Spontaneous changes of form may occur in nuclei. Thus both Flemming and 

 Strieker have described, in preparations of blood, nuclei, which had been set free 

 by rupture of the corpuscles to which they belonged, exhibiting spontaneous changes 

 of form, and Klein has made a similar observation upon the large nuclei of the 

 glands in the skin of the triton. Within the living epithelium-cells of the 

 tail of salamander-larvas, Flemming also noticed alterations taking place in 

 the outline of the nuclei. Several observers have remarked spontaneous changes 

 of form in nucleoli, which they have compared to the amoeboid movements of 

 protoplasm. 



Although most so-called " resting " nuclei, i.e., nuclei which are not at the time 

 in process of division by karyokinesis, exhibit the well-marked reticular appearance 

 just mentioned, some are occasionally met with in which the nuclear filaments are 



Fig. 203. GLAND-CELL FROM A LARVA OF NEMOCERA. (Carnoy.) 

 m, cell-membrane ; p, protoplasm ; ., nucleus with convoluted filament. 



not united with one another to form a network, but have the 

 appearance of a convoluted skein (fig. 208), which may either con- 

 sist of a tortuous filament or of several distinct looped and con- 

 torted threads. When thus formed of a number of looped filaments 

 it may frequently be noticed that one side or pole of the nucleus 

 is left clear of the filaments, the loops of which are set around this pole (fig. 209, A), 

 while their free ends interdigitate at the opposite pole (antipole, fig. 209, B). It is 

 thought probable by Rabl, who has pointed out these differences at the two poles of 

 the nucleus, that this condition is really the typical one, and that it occurs in most 

 nuclei, but that the primary loops of chromoplasm send out secondary twigs in all 



