CONDITIONS OF ANIMAL LIFE 9 



8. Cells. If we examine very carefully the different parts 

 of some highly developed animal under the high powers of 

 the microscope we find that they are composed of a multi- 

 tude of small structures which bear the same relations to 

 the various organs that bricks or stones do to a wall ; and 

 if the investigation were continued it would be found that 

 every organism is composed of one or more of these lesser 

 elements which bear the name of cells. In size they vary 

 exceedingly, and their shapes are most diverse, but, despite 

 these differences, it will be seen that all exhibit a certain 

 general resemblance one to the other. 



9. Shape of cells. In many of the simpler organisms the 

 component cells are jelly-like masses of a more or less 

 spherical form, but as we ascend the scale of life the condi- 

 tion of affairs becomes much more complex. In the mus- 

 cles the cells are long and slender (Fig. 1, D) ; those form- 

 ing the nerves and conveying sensations to and from all 

 parts of the body, like an extensive telegraph system, are 

 excessively delicate and thread-like ; in the skin, and lining 

 many cavities of the body, where the cells are united into 

 extensive sheets, they range in shape from high and colum^ 

 nar to flat and scale-like forms (Fig. 1, E, F, G). The cells 

 of the blood present another type (Fig. 1, B) ; and so we 

 might pass in review other parts of the body, and con- 

 tinue our studies with other groups of animals, always find- 

 ing new forms dependent upon the part they play in the 

 organism. 



10. Size of cells. Also in the matter of size the greatest 

 variations exist. Some of the smallest cells measure less 

 than one micromillimeter ( 2g } 00 of an inch) in diameter. 

 Over five hundred million such bodies could be readily 

 stowed away into a hollow sphere the size of the letter be- 

 ginning this sentence. In a drop of human blood of the 

 same size, between four and five million blood-cells or cor- 

 puscles float. And from this extreme all sizes exist up to 

 those with a diameter of 2.5 or 5 c.m. (one or two inches), 



