32 COMPAEATIVE ZOOLOGY. 



tlie cell -contents." The nucleus genenilly lies near the 

 centre of the protoplasm, and is the centre of activity. 



Cells vary greatly in size, but are generally invisible to 

 the naked eye, ranging from -j^r ^^o looou oi an inch in 

 diameter. About 4000 of the smallest would be necessary 

 to cover the dot of this letter i. The natural form of iso- 

 lated cells is spherical ; but when they crowd each other, 

 as seen in bone, cartilage, and muscle, their outlines be- 

 come angular, either hexagonal or irregular. 



Within the narrow boundary of a simple sphere, the 

 cell-membrane, are exhibited all the essential phenomena 

 of life — growth, development, and reproduction. The 

 physiology of those minute organisms is of peculiar inter- 

 est, since all aniinal structure is but the multiplication of 

 the cell as a unit, and the whole life of an animal is that 

 of the cells which compose it: in them and by them all 

 its vital processes are carried on." 



The structure of an animal cell can be seen in blood- 

 corpuscles, by diluting with a weak (.5 per cent.) solution 

 of salt a drop of blood from a FroS:, and placing it under 

 the microscope. (See Fig. 63.) With this may be com- 

 pared vegetable cells as seen in a drop of fluid yeast or a 

 drop of water into which pollen grains from some flower 

 have been dusted. 



2. Tissues. — There are organisms of the lowest grade 

 (as Paramecium) which consist of a single cell, living for 

 and by itself. In tiiis case, the animal and cell are identi- 

 cal: the Parameckim is as truly an individual as the Ele- 

 phant. But all animals, save these unicellular beings, arc 

 mainly aggregations of cells : for the various parts of a 

 body are not only separable by the knife into bones, muscles, 

 nerves, etc., but these are susceptible of a finer analysis by 

 the microscope, which sliows that they arise from the devel- 

 opment and union of cells. These cellular fabrics, called 

 tissues, differ from one another both chemically and struct- 

 urally, but agree in being permeable to liquids— a property 



