LIVING SUBSTANCE 



75 



cells are so manifold that it is often difficult for the inexperienced 

 observer to realise that they are all different modifications of one 

 and the same type. In contradistinction to this endless variety 

 there exists a wide-spread constancy in the form of one and the 

 same kind of cell, so that the cells of any particular tissue of 

 the human body, e.g., the liver, the skin, the bone, or the blood, 

 are always to be recognised at once as such, i.e., as liver-, skin-, 

 bone-, or blood-cells. A few examples will best illustrate the great 

 differences in the forms of cells. 



There are many cells that possess no constant form, but change 

 their shape continually, and hence are termed amceboid cells. All 

 amoeboid cells have a naked protoplasmic body, upon the surface 



FIG. 16. Amoeba, showing successively different shapes in creeping ; the hyaline exoplasm flows 

 constantly forward ; in the middle and behind lies the granular endoplasm, containing the 

 (darker) nucleus and the (lighter) vacuole. 



of which projections of the body-substance constantly appear and 

 disappear, and thus a new shape is constantly being assumed. In 

 different kinds of cells these projections or pseudopodia have 

 different forms. Most fresh-water Amoebce (Fig. 16) and the egg- 

 cells (Fig. 17, a) of many animals are characterised by broad, lobate 

 or finger-shaped pseudopodia; leucocytes (Fig. 17, &), or colour- 

 less blood-cells, by pointed and divided pseudopodia ; and many 

 Rhizopoda (Fig. 17, c) and pigment-cells (Fig. 17, d) by thread- 

 like and reticulate pseudopodia flowing into one another. 



But by far the majority of cells possess a constant form, whether 

 the protoplasm is enclosed in a membrane or not. The simplest 

 form of cell that can be regarded as the type of the elementary 



