MORPHOLOGY OF THE CELL 29 



products of this new physiology made possible by the cell theory. He- 

 redity could now become a cell problem, and as is pointed out in a later 

 chapter the chief discussion of heredity now has to do with the machinery 

 by means of which cells produce hereditary characteristics. Not all of 

 these developments of zoological investigation took place at once, after 

 the cell theory was published, but they became possible, and a beginning 

 was made. In numerous other ways the outlook of biology was changed 

 and illumined by Schleiden and Schwann's epoch-making law. But 

 in order to understand the influence of the cell theory on the development 

 of the science, it will first be necessary to consider the structure of cells. 



The Size of Cells. — All cells are composed of viscous or jelly-like 

 material called protoplasm. The quantity of this material comprised in 

 a single cell varies within wide limits. Many cells are so small as to be 

 visible only with a good microscope. The vast majority of cells require 

 considerable magnification to be seen. Not a few, however, are large 

 enough to be visible with the unaided eye, as in the case of the larger 

 Protozoa (animals composed of but one cell), while in muscles and nerves 

 of the higher animals, the cells, though not of great diameter, may be 

 inches or feet in length. This great variabiHty does not exist, however, 

 among cells of the same kind. The cells of the skin of a given animal, 

 though not of uniform size, do not differ strikingly from one another; 

 and cells that are arranged in definite layers, like that lining the intestine, 

 are apt to be very nearly equal in size. 



Gross Shape. — The shape of cells is also very variable. Theoreti- 

 cally, due to surface tension, a cell is typically spherical; but that shape is 

 attained, even approximately, only in certain free cells, such as eggs and 

 a few of the one-celled organisms, the Protozoa (animals) and Proto- 

 phyta (plants). Cells take on other forms for various reasons. A free- 

 living cell, as Amoeba and other related Protozoa, may actively change 

 its shape by thrusting out portions of the body into finger-like pseudo- 

 podia. Fig. 17. Such an animal is seldom of the same shape for any con- 

 siderable time (unless it goes into a "resting" state in which it is 

 apt to be nearly spherical) and it may even be changing every instant. 

 Other free-living cells, of more or less constant form, are kept constant 

 by a wall or pellicle that the cells themselves have secreted. These 

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

 may become temporarily distorted it is characteristic of the species. 

 Good examples of such constant forms are found among the Infusoria 

 (ciUated Protozoa), as in Fig. 18. 



Cells that exist in aggregates 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. 

 But other factors, such as unequal growth in different directions, and 

 perhaps inequahties of surface tension, factors often not understood, com- 



