GENERAL ZOOLOGY 



recognized as the directive agents of metabolism which confer uniqueness on 

 both the individual and his species. Synthesis of precise replicas of these 

 compounds is the means by which continuity of the orderly pattern of cel- 

 lular events is insured within an individual as well as between generations. 

 Both nucleus and cytosome are necessary for the normal activities of the 

 cell, but it is not yet known exactly what part each plays in the metabolism of 

 the whole. Cells that are deprived of their nuclei are unable to carry on 

 anabolism, although catabolism continues until the energy reserves are ex- 

 hausted. The nucleus is the seat of nucleic acid synthesis, both DNA and 

 RNA. The DNA located in the genonemata is sometimes referred to as a 

 "set of instructions" for cellular function or as the "memory" of the cell, 

 insuring orderly continuity of life processes. Continuing the interpretation 

 in this vein, RNA, which passes from the nucleus to the endoplasm, functions 

 as a "messenger," carrying instructions from nucleus to cytosome. In experi- 

 ments with the unicellular amoeba (p. 230) from which the nucleus was 

 removed, it was found tliat within 24 hours the RNA content was lowered, 

 and cellular metabolism was defective. These amoebas continued to live for 

 a number of days, and if a fresh nucleus was inserted within three days, 

 normal conditions were restored. It cannot be doubted that there is very 

 close interdependence between the two morphological divisions and that the 

 life of the cell depends on integrated interaction between nucleus and 

 cytosome. 



The Chemical Nature of Protoplasm^ 



Although Thomas Huxley in his famous essay "On the Physical Basis of 

 Life" (1868) discussed protoplasm as though it were a living chemical com- 

 pound, and although the phraseology which uses "living matter" as syn- 

 onymous with the contents of cells carries the same implication, it is now 

 known that protoplasm is not a single compound. Rather, it is a highly com- 

 plex system containing a great number of chemical compounds. 



The most abundant single compound in the cell is water, which is, of 

 course, a widely distributed inorganic compound. Present, too, are many 

 familiar inorganic salts. The compounds that are unique for cells are known 

 as organic compounds and are obtained naturally only from animals and 

 plants. Characteristically, such compounds are combustible; that is, they 

 break down into their constituent parts during burning, and all contain car- 

 bon. Most of them also contain hydrogen and a large number oxygen as well. 



'Functions of living organisms cannot be explained without recourse to information which has 

 been gained about the chemistry of cells. The basic chemistry of the cell will be explained in 

 the simplest terms consistent with accuracy. Since the chemical backgrounds of students regis- 

 tered in an elementary course in zoology will usually vary greatly, it is assumed that some may 

 have to consult elementary textbooks in chemistry or get help concerning sjjecific points from 

 their instructors. 



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