CHAPTER XI 

 CELL DIVISION AND GAMETOGENESIS 



The cell seems to be the smallest or simplest aggregate of 

 matter capable of maintaining itself alone, or in combination with 

 other cells, as a living thing over a period of time, and capable also 

 of reproducing itself. A basic fact in the organization of the cell 

 seems to be that it is always composed of two kinds of protoplasm, 

 nuclear and cytoplasmic. In metazoans cell boundaries may be 

 indistinct or lacking, but every cell region is provided with a 

 nucleus. It is true, of course, that bacteria lack a morphological 

 nucleus, but they do contain nuclear material, even though it is 

 not segregated inside of a nuclear membrane. Mammalian 

 erythrocytes are not nucleated, but these structures are really 

 enucleated cells that survive for a relatively short time and are 

 incapable of reproducing themselves. Since it has also been 

 shown that under experimental conditions neither nucleus nor 

 cytoplasm can survive long in the absence of the other, it appears 

 that the chemicophysical system represented by the nucleus and 

 cytoplasm combined as in a cell is the minimum for the produc- 

 tion and continuation of the living state. 



It is a dictum of biology that cells come only from preexisting 

 cells by a process of cell division; and, on the other hand it is also 

 said that the first forms of life, or the first cells, to appear on the 

 earth were produced from nonliving matter (abiogenesis). No 

 one can say what these first forms of life were like, or to what 

 extent they resembled the simplest kinds of living things known 

 today. Whatever may be the answer to that problem, it has 

 been conclusively shown that known present-day species of living 

 things come from preexisting species, or that the cells of which 

 they are composed come from preexisting cells. Reproduction of 

 organisms depends upon the reproduction of cells, which in turn 

 is the result of cell division. Cell division is thus a very impor- 

 tant phase of biological activity (Fig. 136). 



The cells of the body of the metazoan animal fall into two 

 general groups, somatic cells and germ cells, although these two 



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