lO 



Cell-divisiofi 



lI2/ith the process of celi>-division, the fertilized 

 or parthenogenetic egg, a single cell, develops into an adult 

 organism which may comprise millions of cells. The first 

 division or cleavage separates the egg (in most cases) into 

 two portions, blastomeres, which remain attached. Suc- 

 cessive cleavages give rise to many blastomeres, the egg 

 becoming a mass of cohering cells. The surface of the 

 frog's egg, for example, in late cleavage so closely resembles 

 that of a golf-ball that one gains the impression that sub- 

 division of its surface by cleavage-planes is the egg's chief 

 characteristic. If one examines a smaller egg, e.g., of a 

 sea-urchin, under the microscope, one notes again the 

 resemblance of its corrugated surface to a golf-ball; looking 

 closer, one observes within each blastomere a nucleus. 

 It is a very simple matter to convince oneself by continuous 

 observation on a living egg throughout its cleavage period 

 that the "golf-ball" has arisen by successive divisions of 

 both nucleus and cytoplasm. 



The cells which comprise an adult organism are not all 

 alike in structure. To the naked eye a strand of nerve is 

 readily distinguishable from a strand of muscle of equal 

 length and thickness; and a piece of kidney can not be mis- 

 taken for liver. As en masse, so singly, under the micro- 

 scope, the cells of nerve, muscle, kidney, and liver are easily 

 recognized. Since these cells have different functions, 

 development means something more than mere multiplica- 

 tion of cells; it implies the cells' differentiation. This 

 arises in the process of cell-multiplication at an earlier or 



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