24 



PRINCIPLES OF ANIMAL BIOLOGY 



functions, but in most cases no such explanation is known. When cells 

 that are presumably alike in their origin and function show great differ- 

 ences in volume, as when one unicellular animal (Paramecium, for 

 example) is several hundred times as large as another 

 of the same species (Fig. 15), it is probable that 

 differences in the environment have caused part, though 

 not all, of the contrast. 



The size of cells bears no constant relation to the 

 size of the animals or plants in which they are found. 

 In very many kinds of animals, large individuals have 

 more, but not larger, cells than do small ones. In 

 others, the number of cells in each individual is 

 always the same, and in them large size is attained 

 only by the growth of each cell. In salamanders in 

 which, through some abnormal step in cell division, 

 the cells have extra chromosomes, the cells are larger 

 but the body is not: such animals simply have fewer 

 cells. Sluggish animals like frogs generally have larger 

 cells than active ones such as birds, and there is 

 presumably some important connection between these 

 facts. 

 Gross Shape. — The shape of cells is also very variable. Some cells, 

 owing to surface tension, are typically spherical; but that shape is 

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

 of the one-celled organisms. Cells take on other forms for various 

 reasons. Amoeba and other related protozoa may actively change their 



Fig. 15. — Ex- 

 treme difference of 

 fsize in otherwise 

 similar cells; two 

 members of same 

 species of Para- 

 mecium, one 300 

 times as large as 

 the other. 



Fig. 16. — Change of shape in amoeba. Half-ininuto interval between first and second, 

 five mjnutes between second and third. {CoiirUsy of Gcnenil Biological Supply House.) 



shape by thrusting out portions of the body into fingerlike pseudopodia. 

 Such an animal is seldom of the same shape for any considerable 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 (Fig. 16). Other 

 free-living cells, of more or less constant form, are kept constant by a wall 

 or pellicle that the cells themselves have secreted (Fig. 17). These 



