PRIMARY ORGANIZATION OF LIVING MATTER 



25 



pellicles may be flexible but firm, so that while the shape of the body 

 may become temporarily distorted it is characteristic of the species. 



Cells that exist in groups usually have their form altered by the 

 mechanical pressure of the cells around them. When this pressure is 

 the only factor altering their shapes, the cells are irregular polyhedrons. 

 Other factors, such as unequal growth in different directions and perhaps 

 inequalities of surface tension, combine to produce cells of a great variety 

 of shapes. They may be box-shaped, as in plants; long cylinders, as in 

 voluntary muscle; greatly flattened cells with their largest sides polygons, 

 as in the outer layer of frog skin; somewhat flattened elliptical cells, as in 

 the blood of many animals; circular and flattened, as in human blood; 

 narrow and spindle-shaped, as in involuntary muscle; or finely branched, 



'i«Aj ^"-~_-^-^^^==^ 



Fig. 17. Fig. 18. 



Fig. 17. — Various forms of ciliated protozoa whose body shape is kept fairly constant 

 by a surrounding pelhcle. Though this shape may be altered by pressure, it is restored 

 when the pressure is removed. Cilia project from the surface. 



Fig. 18. — Various forms of nuclei in cells. A, part of muscle cell with multiple ellip- 

 soidal nuclei; B, gland cell of butterfly with branching nucleus; C, marrow cell of rabbit with 

 ring nucleus; D, Epistylis with curved rodlike nucleus; E, Stentor with beaded nucleus; 

 F, Trachelocerca with distributed nucleus. (5, C, and F after Wilson, courtesy of The Mac- 

 Millan Company.) 



as in pigment cells of the skin of frogs and salamanders, or bone and nerve 

 cells. 



The Nucleus. — The most important part of a cell is its nucleus. 

 This body is ordinarily located somewhere near the middle of the cyto- 

 some but may be crowded to one side by other structures and may move 

 from one place to another. It is most often spherical, owing to the ten- 

 sion of the very thin film, or nuclear membrane, which surrounds it, but 

 other shapes may be impressed upon it or it may actively take other 

 forms. In long narrow cells the nucleus is generally elongated (Fig. 

 18A), and in flat cells it is disk-shaped. Physiologically very active 

 cells often have branched or lobed nuclei {B, C) ; and in certain unicellular 

 organisms the nuclei may be of odd shapes — ropelike, beaded, or broken 

 up into many small bits (D-F) — characteristic of the species but without 

 any known significance. The red cells of human blood are devoid of 

 nuclei, a condition generally held to be due to degeneration of the nuclei 

 which they possessed in young stages. 



