GENERAL ZOOLOGY 



from 1 to 2 hours under favorable nutrient conditions. There follows a period 

 of the activity characteristic of the particular kind of cell, and after about 

 12 hours mitosis may occur again, followed by cytosomal constriction. The 

 times given here are for cells in tissue cultures; other kinds probably differ 

 somewhat, and a variety of bodily conditions doubtless affect division rates. 

 It should be clear that what evidence exists indicates that the chromatic 

 threads found in each anaphase and telophase chromosome persist through 

 the metabolic phase to the succeeding prophase, even though there is no 

 morphological proof. The importance of this continuity of the genonemata 

 from cell generation to cell generation will be increasingly apparent, and the 

 changes that occur during mitosis should be clearly understood. 



Summary 



In this chapter we have described the structure of the cells which make up 

 the bodies of animals and in which the vital processes occur. As we study 

 many-celled animals, we shall find that major functions are associated with 

 special structures — digestion with a digestive tract and glands, gas exchange 

 with gills or lungs or air tubes, circulation with pulsating tubes and special 

 fluids. Structure and function are intimately related to the organism as a 

 whole, and so it is with cells. 



One of the things you may have thought about as you studied the "com- 

 posite cell" is that all the different parts are definitely located. Many of 

 the structures are separated from one another by membranes, just as the en- 

 tire cell is separated from other cells by its plasma membrane. These 

 membranes are semipermeable, however, and some are porous in addition — 

 communication is possible between the various compartments of the cell. 



It has been known for a long time that the chromosomes found in the 

 nucleus of the dividing cell are the structures associated with the synthesis 

 and transmission of the hereditary material of the cell, and so of the individual 

 and species. In recent years, other functions of the cell have been associated 

 with particular structures. Elucidation of the steps in cellular metabolism 

 reveals that enzymes work as teams in carrying out the transformations of 

 metabolism. And the inference is made that very often, perhaps always, the 

 members of these teams occupy definite positions with reference to one an- 

 other; space relations are important in chemical reactions. Insoluble enzymes 

 involved in cellular oxidation are known to be located on the mitochondrial 

 membrane; soluble ones are in its matrix. 



The great expanse of lipoprotein membrane in the endoplasmic reticulum, 

 together with the obvious possibility for precise spacing of enzymes on it — 

 like traffic directives painted on a highway — and its typical association with 



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