THE ORIGIN OF LIFE 137 



cumstances). Normally, therefore, the tissue cell is function- 

 ally incomplete, a part and not a whole, whereas the protist 

 is an independent individual, being, at once, the highest type 

 of cell and the lowest type of organism. 



According to the classic definition of Franz Leydig and Max 

 Schultze, the cell is a mass of protoplasm containing a nucleus, 

 both protoplasm and nucleus arising through division of the 

 corresponding elements of a preexistent cell. In this form the 

 definition is quite general and applies to all cells, whether 

 tissue cells, germ cells, or unicellular organisms. Moreover, it 

 embodies two principles which still further determine the law 

 of genetic cellular continuity, namely: Omnis cellula ex cellula, 

 enunciated by Virchow in 1855, and Flemming's principle: 

 Omnis nucleus ex nucleo, proclaimed in 1882. In this way, 

 Cytology supplemented Redi's formula that every living being 

 is from a preexistent living being, by adding two more ar- 

 ticles, namely, that every living cell is from a preexistent cell, 

 and every new cellular nucleus is derived by division from a 

 preexistent cellular nucleus. Now neither the nucleus nor the 

 cell-body (the cytoplasm or extranuclear area of the cell) is 

 capable of an independent existence. The cytoplasm of the 

 severed nerve fibre, when it fails to reestablish its connection 

 with the neuron nucleus, degenerates. The enucleated amceba, 

 though capable of such vital functions as depend upon destruc- 

 tive metabolism, can do nothing which involves constructive 

 metabolism, and is, therefore, doomed to perish. The sperm 

 cell, which is a nucleus that has sloughed off most of its cyto- 

 plasm, disintegrates, unless it regains a haven in the cytoplasm 

 of the egg. Life, accordingly, cannot subsist in a unit more 

 simply organized than the cell. No organism lives which is 

 simpler than the cell, and the origin of all higher forms of life 

 is reducible, as we shall see, to the origin of the cell. Conse- 

 quently, new life can originate in no other way than by a 

 process of cell-division. All generation or reproduction of new 

 life is dependent upon the division of the cell-body and nucleus 

 of a preexistent living cell. 



