78 



INTRODUCTION TO CYTOLOGY 



successive eell generations. They observed that, prior to cell-division, 

 the centrosome divides to form two daughter centrosomes, which move 

 apart to opposite sides of the cell and form the poles between which the 

 mitotic figure is established; and further, that after cell-division is 

 completed the centrosome included in each daughter cell does not 

 disappear, but remains visible in the cytoplasm through the ensuing- 

 resting stage. Because of this striking behavior at the time of cell- 

 division (see further p. 177) the centrosome soon came to be known as 

 "the dynamic center of the cell." 



The above facts seemed to constitute ample ground for the conception 

 of the centrosome as a permanent cell organ, but many obstacles have 

 been found in the way of its acceptance as a theory of universal applica- 

 tion. At certain stages in the history of many animal cells its presence 



-'■ ■-■.- ... \ ■•;; — ^'-4|$8 OxO.- 



Fig. 20. — Artificial cytasters in the egg of Arbacia. (After Morgan, 1899.) 



cannot be demonstrated, and it is entirely absent from the cells of higher 

 plants. Furthermore, Mead (1898) and Morgan (1896. 1898) found that 

 the formation of centrosomes with asters may be induced in the eggs of 

 certain animals by artifical means (treatment with NaCl and MgCl 2 

 solutions) (Fig. 20), and it has been claimed that centrosomes so formed 

 may function normally in the ensuing division (cleavage) of the egg. 

 Contrary to the opinion of Boveri (1901), Wilson (1901) regarded such 

 "artificial cytasters" as true asters with true centrosomes. Conklin 

 (1912), however/contends that they do not function in mitosis. It is 

 probable that no single conclusion can be drawn concerning this matter 

 which will apply to all cases. There seems to be good evidence for the 

 view that the centrosome in some tissues exists as a permanent cell organ, 

 dividing at each mitosis and remaining visible through the resting stages, 



