AN INTRODUCTION TO THE 

 STUDY OF CYTOLOGY 



CHAPTER I 



The Cell 



(YTOLOGY may be defined as the study of cells; it 

 differs from the related science of Histology in dealing 

 with cells as units, while histology is concerned with cells in 

 larger aggregates, and is rather the study of tissues. A 

 serious defect, however, in this definition of cytology is that 

 it involves the term "cell," a word which at once opens up 

 a wide field of controversy and misunderstanding. As long 

 as it was generally agreed that all organisms are built up of 

 cells much as a house is built of bricks, the description of 

 the cell as "a unit of living matter" was not open to any 

 very grave objection; the cell was to the biologist almost 

 what the atom was to the chemist the smallest portion of 

 living matter capable of an independent existence and the 

 word had, in appearance at least, a fairly definite meaning. 

 Now, however, when the old idea of discrete and indepen- 

 dent cells is almost abandoned, and when distinguished 

 biologists maintain that one whole group of organisms (the 

 Protista) are non-cellular, the word "cell" is beginning to 

 lose its definite and precise significance, and to be used 

 rather as a convenient descriptive term than as denoting 

 a fundamental concept of biology. 



The cell-theory arose, of course, from observation of the 

 structure of plants. A section of an ordinary plant shows 



D.C. I 



