INTRODUCTION TO CYTOLOGY 



CHAPTER I 



CELLS AND TISSUES 



Every living organism, plant or animal, is a protoplasmic system 

 interacting with its environment. Although it is probable that the 

 protoplasm of every kind of living thing, even every individual, differs 

 physico-chemically from that of every other, it is nevertheless true that in 

 certain major features of its organization protoplasm tends to be strikingly 

 the same throughout almost the entire organic world. With the possible 

 exception of certain lowly types of life, protoplasm everywhere is differ- 

 entiated into two principal constituents, cytoplasm and nucleus. Fur- 

 thermore, in most plants and animals protoplasm is organized in the 

 form of cells, each cell consisting typically and essentially of a nucleus, a 

 mass of cytoplasm, and a limiting membrane. 



Cytology derives its name from the cell: kvtos = hollow place (a cell). 

 "Cytology, the study of cells" is a convenient definition, but in certain 

 respects it is scarcely adequate. There are many tissues and many fairly 

 complex organisms which are really large masses of nucleated protoplasm 

 with actual cellular subdivisions partially or wholly absent, yet they 

 carry on the same vital activities and confront the biologist with nearly 

 all of the same fundamental problems as do the typically cellular forms. 

 Hence it seems better to say that cytology is that branch of biology which 

 deals directly with the structural and functional organization of proto- 

 plasm, usually in the form of cells, and with the relation of this organi- 

 zation to such phenomena as metabolism, ontogenetic differentiation, 

 heredity, and phylogeny. 



Throughout most of its historical development cytology concerned 

 itself chiefly with the structural aspects of protoplasmic organization, 

 its principal tool being the microscope. Since the nature of light and 

 the structure of the human eye impose arbitrary limitations upon the 

 applicability of microscopical methods, structural cytology came to be 

 rather sharply set apart from the study of physiological function, in which 

 a different set of techniques had to be employed. The artificial character 

 of this distinction should be obvious. Every structure has its functional 

 meaning, while every physiological proce.ss involves a structural alteration 

 of some order in the protoplasmic system. Modern cytology is making 



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