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COMPARATIVE ZOOLOGY 



common to the whole organism, is handed over to the 

 nerves. An animal, then, whose body, instead of being 

 uniform throughout, is made up of different parts for the 

 performance of particular functions, is said to be organ- 

 ized. And the term is as applicable to the slightly dif- 

 ferentiated cell as to complex man. Organization is 

 expressed by single cells, or by their combination into 

 tissues and organs. 



i. Cells. A cell is the simplest form of organized 

 life. In general, it is a microscopic globule, consisting 



FIG. igB. A, diagram of a cell; TV, cell wall, with inclosed cytoplasm; , nucleus, 

 consisting of nuclear membrane inclosing granular substance, in which are seen a 

 spherical nucleolus and several irregular masses of chromatin ; a, attraction sphere 

 containing a centrosome. B-F, changes which take place in the cell during fission. 



of a delicate membrane inclosing a minute portion of 

 protoplasm. The very simplest kinds are without gran- 

 ules or signs of circulation ; but usually the protoplasm 

 is granular, and contains a defined separate mass called 

 the nucleus, within which are sometimes seen one or 

 two, rarely more, dark, round specks, named nucleoli. 

 The enveloping membrane is extremely thin, transpar- 

 ent, and structureless ; it is only an excretion of dead 

 matter acting as a boundary to the cell contents. 69 The 

 nucleus generally lies near the center of the protoplasm, 

 and is the center of activity, 



