76 



GENERAL PHYSIOLOGY 



organism is the spherical form, as it appears, for example, in many 

 egg-cells (Fig. 18, a). From this type deviations in all sorts of 

 directions occur. When the cells are united with other similar 

 ones, as is the case in every tissue, their form is modified by 

 the pressure which they receive from the surrounding cells. A 

 cell which in itself is spherical must, therefore, in a tissue take 

 on a polyhedral form according to simple mechanical laws, just as 



FiQ. 17.— a, Bgg-ceU ot a calcareous sponge. (After Haeckel.) 6, Blood-ceU of a crab (After 

 Haeckel.) c, Bioimjxa mgans, a fresh-water rhizopod, d, Pigment-cell from the tail of a 

 tadpole. 



peas lose their spherical shape, and become polyhedral when they 

 are crowded thickly in a bottle and are made to swell. In fact 

 the polyhedral shape of cells occurs very frequently in tissues 

 especially in epithelium-cells of the skin (Fig. 18, I) and gland- 

 cells. Further, one essential factor in causing a deviation from 

 the spherical type is the formation of permanent processes upon 

 the surface. In this way permanent forms of cells often occur 

 of the shape that amceboid cells show temporarily. The green 



