366 LIFE : OUTLINES OF GENERAL BIOLOGY 



of the glass plate almost disappearing before the oil, the chemical 

 coherence or inertia yielding before the catalyst or the ferment. 

 And just as some oils work better than others, so it is with enzymes. 



STRUCTURE AND LIFE OF THE CELL 



The Cell Theory. — As we have seen, not a few of the early 

 microscopists made attempts to define the minute elementary parts 

 that build up living creatures; but it was not till 1838 that the idea 

 of the cell as a structural and functional unit was clearly focused 

 in the Cell Theory, or, better. Cell Doctrine (Zellenlchre) of Schwann 

 and Schlciden. This generalisation includes three projx)sitions. 

 First, there is the morphological statement, that all living creatures 

 have a cellular structure, and that all but the simplest, that is to 



Fig. 47. 



Two animal cells showing protoplasmic intercellular bridges between them 

 After Pfitzner. i, cell boundary; 2, nuclear boundary; 3, nucleoplasm; 

 4, cytoplasm. 



say, all that have what may be called a "body", are built up of 

 cells and modifications of cells. Even a minute "Wheel animalcule" 

 or Rotifer, that can swim through the eye of a needle under the 

 microscope, has nearly a thousand cells. The insertion of "modi- 

 fications of cells" is to covTr cases like a vessel in a plant, which may 

 arise from the fusion of a row of cells, or like a long muscle-fibre in 

 an animal which sometimes consists of several cells intimately 

 united, or again, like the "syncytium" of some embroys (e.g. 

 species of Paripatus), where the boundaries between adjacent cells 

 arc not well-defined. Second, there is the physiological statement, 

 that the activity of a many-celled organism is the; sum of the 

 activities of the com]x>nent cells. This idea requires to be safe- 

 guarded by the fact of correlation, for the life of the whole cannot 

 be described without recognising that it is more than the life of 

 all its parts, just as the behaviour of a crowd with a common pur- 

 pose cannot be adequately described merely in terms of the move- 

 ments of all the individuals. This correlation, elsewhere discussed, 

 is more marked in the animal than in the plant, and most marked 



