MOIU'HOLOGY OF THE CELL 



:n 



in human blood; narrow and spindle-shaped, as in involuntary muscle; or 

 finely branched, as in pigment cells of the skin of frogs and salamanders, 

 or bone and nerve cells. 



Nucleus and Cytoplasm. — This mass of protoplasm, regardless of 

 its shape and other modifications, is usually differentiated into two dis- 

 tinct parts. There is a central body called the nucleus (Fig. 20, n) 

 which, in prepared and stained material, is usually highly stained, since 

 it is composed of substances which have a high affinity for most of the 

 common dyes; but in living material the nucleus is somewhat transparent, 

 nearly colorless, and highly refractive like a lens. The remainder of 

 the cell, besides the nucleus, is called cytoplasm (Fig. 20, cy). Both 



Fig. 19. — Various forms of nuclei in cells. A, part of muscle cell with multiple el- 

 lipsoidal nuclei; B, glaud cell of butterfly with branching nucleus; C, marrow cell of rabbit 

 with ring-nucleus; D, Epistylis with curved rod-like nucleus; E, Stentor with beaded 

 nucleus; F, Trachelocerca with distributed nucleus. {B, C, F, after Wilson. Courtesy 

 of Macmillan Co.) 



nucleus and cytoplasm are composed of protoplasm, as the latter term 

 is now commonly used. 



The nucleus is typically spherical, owing, as in the case of the cell as a 

 whole, to surface tension. But this shape is often altered by the shape 

 of the containing cell, by physiological activity of the nucleus itself, or 

 for reasons not yet understood. In long narrow cells the nucleus is apt 

 to be ovoid or ellipsoidal, as in young voluntary muscle cells (Fig. 19, A). 

 In highly active cells it may be branched {B) or otherwise irregular 

 (ring-shaped, C). In some of the Protozoa it is rope-like (D), or beaded 

 {E). There may be more than one nucleus, as in a number of Protozoa, 

 and in voluntary muscle (A). Some cells have no nucleus of the usual 

 structure, but possess the deeply staining nuclear material in the form of 

 scattered granules. The bacteria and some of the Protozoa (F) are 



