40 ESSENTIALS OF ZOOLOGY 



body ; Grew had made rather extensive microscopic studies of plants, 

 and in 1831 Robert Brown had discovered the nucleus of the cell, 

 but not until the work of Schleiden in 1838 and Schwann in 1839 

 was the cell theory formally enunciated. The former a botanist and 

 the latter a zoologist, each working independently, came to the same 

 conclusion and in 1839 collaborated their ideas. This theory, as 

 they gave it, was in substance, All living things (plants and animals) 

 are composed of cells. 



It is no discredit to this theory or these men that they and many 

 other biologists of the time had erroneous ideas concerning the 

 essential features of the cell. Although Brown had recently dis- 

 covered the nucleus, the cell wall was thought to be the essential 

 part, though now we know it is not a universal structure of all 

 cells since practically no animal cells have a cell wall. The notions 

 of the origin of cells and the functional significance were almost 

 wholly fantastic, yet the cell theory proved to be such a unifying 

 generalization and inspiring stimulus to investigation that it became 

 the turning point in the development of biological study. 



The bare statement that living beings are composed of cells soon 

 became inadequate as studies of cells progressed. It was soon found 

 that some tissues are made up not only of cellular structures but 

 included also certain noncellular materials produced by the cells. 

 The matrix, so abundant between the cells of cartilage, was soon 

 found to be noncellular and to be produced by the cartilage cells 

 which became embedded in it. This matrix is not strictly living 

 matter since it is inactive and passive as far as life processes are 

 concerned. Connective tissue fibers fall in the same category. Since 

 living bodies are composed of such an abundance of this noncellular 

 material produced by the cells, the cell principle soon came to be 

 stated thus : All living things are composed of cells and cell products. 

 With the years, the conceptions of the nature of the nucleus, the 

 cell membranes, and the composition of protoplasm itself have all 

 added their contributions to the present understanding of the mean- 

 ing and application of the cell principle. The cell is now regarded 

 as a physiological unit as well as a structural one, and as almost 

 a corollary to the original statement of the principle, namely, that 

 the activities of the organism equal the sum of the activities of its 

 cells. 



With the embracing of the functional activity of the cell as a part 

 of the principle underlying living processes, comes also the inclusion 



