CHAPTER II 

 THE CELL AND CELL DIVISION 



Two universal characteristics of living things are the posses- 

 sion of protoplasm and a cellular composition. Recognition of 

 these fundamental facts was dependent upon the use of the 

 compound microscope, and so we find them comparatively 

 late acquirements in the history of biology. The cell-unit 

 structure of an organic tissue was first described in plants 

 (cork tissue), by. Robert Hooke in 1665, and quite naturally, 

 therefore, emphasis was laid upon what we now know as the 

 "cell walls." As a consequence the term "cell" was applied 

 to these small box-like units which seemed to resemble the 

 cells of a honey comb. When, about the middle of the nine- 

 teenth century, it became apparent that the cell content, and 

 not the cell wall, was the important thing, the word "cell" had 

 become so definitely fixed that it could not but be retained, 

 although its utter inaptness was fully recognized. 



This is not the place to discuss the general importance and 

 significance of the Cell Theory of Schleiden and Schwann (1839) 

 and their successors. It will become clear as we proceed that 

 the cell, in structure and in action, is the basis of modern 

 Embryology. Most of the early processes of organic develop- 

 ment are strictly cell processes and must be studied from the 

 standpoints of both Cytology and Embryology, from neither 

 alone, and throughout development constant reference must 

 be had to cellular phenomena. 



As known to-day cells of different organisms and different 

 tissues exhibit an unending variety in size, frfrm, structure, and 

 function (Figs. 14, 15), but throughout there are two essentials 

 of structure expressed by the definition of a cell given by Leydig 

 (1852) and by Schultze (1861) as "a mass of protoplasm con- 



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