8 ANIMAL FORMS 



The cells of the blood present another type (Fig. 4, 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. 



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

 variations exist. Some of the smallest cells measure less 

 than one micromillimeter (^TO-^ 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), 

 as in the case of the hen's or ostrich's egg. On the average 

 a cell will measure between .025 to .031 m.m. ( TTi Vfr an( ^ 

 -g-J-o of an inch) in diameter, a speck probably invisible to 

 the unaided eye. While the size and external appearance 

 of a cell are seen to be most variable, the internal structures 

 are found to show a striking resemblance throughout. All 

 are constructed upon essentially the same plan. Differ- 

 ences in form and size are superficial, and in passing to a 

 more careful study of one cell we gain a knowledge of the 

 important features of all. 



10. A typical cell. Some cell, for example that of the 

 liver (Fig. 4, A), may be chosen as a good representative of 

 a typical cell. To the naked eye it is barely visible as a 

 minute speck ; but under the microscope the appearance is 

 that of so much white of egg, an almost transparent jelly- 

 like mass bearing upon its outer surface a thin structure- 

 less membrane that serves to preserve its general shape and 

 also to protect the delicate cell material within. The com- 

 parison of the latter substance to egg albumen can be car- 

 ried no further than the simple physical appearance, for 

 albumen belongs to that great class of substances which 

 are said to be non-living or dead, while the cell material 



