12 FUNDAMENTALS OF CYTOLOGY 



between C3rtoplasm, nucleus, and environment. Within the ty])ical pro- 

 toplast the localization of certain reactions in particular regions goes hand 

 in hand with structural differentiations, much as it does at a higher level 

 of organization in the various regions of a large and complex multi- 

 cellular body. 



As will be realized more fully later on, the concept of the cell as a 

 fundamental organic unit has occupied a large place in biological thought. 

 (1) It has afforded the student of organic structure a convenient con- 

 crete unit for descriptions of the minute anatomy and development of 

 plants and animals. (2) The physiologist has often been able to gain 

 insight into the functional activities of complex organisms by studying 

 the activities of single cells. (3) The fact that many minute organisms 

 have the structure of a single cell has stimulated speculation, much of it 

 doubtless profitable, on the problem of the evolution of larger organisms. 

 We therefore begin our cytological studies with the typical cell before us, 

 even though our conceptions of its role may become modified as we 

 proceed. 



It is a habit of long standing to interpret a large organism in terms 

 of its cell units. We cannot emphasize too strongly the importance of 

 interpreting the behavior of the cell in terms of the complete organism 

 of which it may be a small part. The relationship existing between the 

 two individualities — the multicellular organism and the unit cell — can 

 be made clearer through a consideration of the various arrangements 

 assumed by cytoplasm, nuclei, and limiting membranes in particular 

 tissues and complete organisms and by observing how these arrange- 

 ments may change into one another as the development of the organism 

 progresses. 



Ontogeny, the development of an elaborate individual animal or 

 plant from a fertilized egg, a spore, or any other small and relatively 

 simple initial mass of protoplasm, is surely one of the most amazing 

 phenomena on earth. The seemingly simple initial protoplast has, 

 however, its own peculiar microscopic and submicroscopic organization. 

 It is a major task of biologists to describe this organization and to show 

 how it functions in the gradual transformation of the initial protoplast 

 into a mature organism with its higher degrees of organization. This 

 transformation involves almost innumerable structural changes and 

 functional reactions, known and unknown. Our immediate purpose is 

 not to describe any of these in detail, but rather to deal in a more general 

 way with certain fundamental aspects of ontogeny, viz., growth, differen- 

 tiation, and correlation. We shall then be in a better position to appre- 

 ciate the relationship between the organism as a whole and the cell. 



Growth. — Growth consists primarily in the synthesis of new proto- 

 plasm through the activity of that already present. It is ordinarily 



