MORPHOLOGY OF THE CELL 27 



early discovered that certain tissues are made up in part of structures and 

 material that are not cellular, but that have been created by cells. The 

 bulk of cartilage and bone is formed of a firm matrix not composed of 

 cells, but lying between the cells. This matrix is produced by the cells 

 as a secretion, in which the producing cells become imbedded. It is not 

 changeless, since the cells may dissolve it away and build it in some other 

 direction, but it is itself wholly inactive in this change. The matrix is 

 not living matter. Connective tissue also contains elastic matter, in 

 the form of fibers of several kinds, which are laid down as secretions by 

 the cells. These fibers have a function, but it is entirely a passive one. 

 Ligaments and tendons are in like manner largely non-cellular. Since 

 animal bodies are so largely composed of such non-living material, all 

 of it produced by cells, the cell doctrine should be made to specify that 

 all living things are composed of cells or cell products. 



With the corrections regarding the nature of nucleus, walls, and proto- 

 plasm, and the addition of cell products, noted in the preceding para- 

 graphs the early statement of the cell doctrine would still be essentially 

 correct as far as it goes. In that form, however, it would imply that biology 

 is mostly a morphological subject. So it once was. But with the growth 

 of the science, with its development in the fields of physiology and genetics, 

 it becomes apparent that the cell doctrine can be extended to include more 

 than the mere structure of animals and plants. 



The life processes of living beings, fortunately now fairly well under- 

 stood in some respects, have been shown more and more to be the fife 

 processes of their component cells. Without implying that the cells 

 act independently of one another, it is still in a sense true that the activities 

 of an organism are the sum of activities of its cells. Under favorable 

 cultural conditions, cells may long continue their activities when removed 

 from their association with other cells; and in many cases it has been 

 possible to analyze the complicated processes of multicellular organs, and 

 show that this thing is done by certain cells, that other thing by certain 

 other cells, and so on. The cell has now come to be regarded as the 

 physiological unit, as well as the morphological one ; and some of the most 

 important advances is zoological knowledge in recent years have been 

 made in the field of cell physiology. 



Almost as a part of the step by which the cell doctrine is extended 

 to embrace cell physiology as one of its tenets, is the inclusion of develop- 

 ment and heredity. The origin of a multicellular animal from a 

 one-celled beginning (the egg) may be watched from first to last, and 

 everywhere the process is a series of cell phenomena. Cell divisions, 

 arrangement of cells into layers or tissues, the unequal growth of cells or 

 parts of cells to form folds and protrusions, the migration of cells some- 

 times for long distances, the formation here and there of cell products, 



