LIVING SUBSTANCE 21 



less material, whether inorganic or organic, by its properties of irritabil- 

 ity, conductivity, contractility, metabolism and reproduction. 



The Structural Basis of Life. — While living substance appears in 

 many forms, it always presents itself as an entity which is capable of 

 leading an independent existence. It is living organic material and 

 as such is generally arranged in the form of cells. In a general way, it 

 may be said that this term is applied to the smallest particles of living 

 substance still capable of existing independently of others. Hence, 

 the cell represents the simplest type of individuality of living substance 

 and constitutes a unit in structure as well as in function. 



It is true, however, that our conception of a cell is not at all concise, 

 because cells may exhibit very different characteristics. To begin 

 with, the term ''cell" was employed by botanists to describe those 

 structural units which make up the stem and the leaves of plants. 

 In a similar way it was found later on that the organs and tissues 

 of the higher animals are not composed of homogeneous masses of 

 living substance, but of a multitude of very small particles which are 

 separated from one another by partitions. In both instances the cell 

 was finally observed to be a definite unit of the entire mass, consisting 

 of a membrane investing a semi-solid globule of protoplasm and a dark 

 body, or nucleus. 



It soon became evident that this conception is not absolutely cor- 

 rect, because the studies of Schultze^ upon the structure of the rhizo- 

 pods proved that there are organisms in existence which are not sur- 

 rounded by a cell membrane, but appear merely as naked masses of 

 living substance possessing the same characteristics as the viscous 

 contents of the plant cell, or protoplasm. In accordance with this 

 discovery, it has since been held that the essential unit of the cell is 

 the protoplasm, i.e., the cell consists merely of a globule of protoplasm 

 which may or may not be invested by a membrane. Our original 

 idea regarding its structure has also been modified in so far as the 

 nucleus is no longer regarded as an essential constituent. This con- 

 ception necessitated a different interpretation of the discovery of 

 Brown^ than that ordinarily given to it. It will be remembered that 

 this investigator noted that protoplasm embraces a granule possessing 

 the power of refracting light. This fact was greatly amplified later 

 on by Schleiden^ and Schwann* who found this granule so universally 

 present that they considered it as a constant constituent of the cell. 

 Hackel,^ however, proved subsequently that many rhizopods do not 

 contain a nucleus. In more recent years this condition has also been 

 shown to prevail in bacteria and fungi. It seems best, however, not 

 to emphasize this point too strongly, because while many cells do not 



^ Archiv fur Anat. und Physiol., 1861. 



2 Transact, of the Linnean Soc, London, 1833. 



3 Miiller's Archiv, 1833. 



^ Mikr. Unters. iiber die Struktur und den Wachstum der Tiere and Pflanzen, 

 1839. 



^Biolog. Studien, Leipzig, 1870. 



