CHAPTER III 



THE UNIT OF STRUCTURE 



IMMEDIATELY after its discovery in the seventeenth century, 

 the compound microscope was applied to the study of minute 

 plants and animals, their organs and tissues. In this connec- 

 tion and for this purpose the microscope has steadily improved, 

 until perfection has almost been attained. Calculations 

 based upon the physical properties of refracting media show 

 that the limits of the assistance which it can give to the eye 

 have been very nearly reached. One of the first results of 

 the application of the microscope to the study of parts of 

 plants was the discovery of their cellular structure. Robert 

 Brown, looking at slices of cork, saw that its tissue is 

 divided into compartments. It is difficult to ascertain who 

 it was that first used the word " cell." The resemblance of 

 a slice of vegetable tissue or the surface view of a petal of a 

 flower to honeycomb is so striking that the same comparison 

 probably occurred to the mind of everyone who saw it. Further 

 study with better instruments showed that the cells are not 

 empty. Each cell contains cell-juice, or cell-substance, and 

 in the centre of the cell-substance a miniature cell, the nucleus. 

 Naturalists therefore extended the connotation of the term. 

 A cell was no longer a space with enclosing walls ; it had 

 contents. A nucleus was invariably a constituent of the cell. 

 The cell was regarded as an anatomical unit, consisting of a 

 wall, cell-contents, and nucleus. In 1839 Theodor Schwann, 

 using his microscope in the study of animal tissues, recognized 

 the similarity between animals and plants. Animals also, he 

 discovered, are aggregations of cells. He enunciated the 

 Cell Theory. Philosophers are always ready to generalize. 

 It is their business. Seeing that vast numbers of organisms 

 are single cells, that they feed, breathe, divide, and reproduce 



