CHAPTER V 

 THE CENTROSOME AND THE BLEPHAROPLAST 



THE CENTROSOME 



For a full description of the morphology and behavior of the centro- 

 some, based upon the large amount of work done on animal cells prior to 

 1900, reference should be made to Wilson's book on the cell. In the 

 present account attention will be devoted mainly to centrosome structures 

 in plants. The centrosomes of animal cells will be described only in 

 general terms, their role in cell-division being dealt with in later chapters. 

 Occurrence and General Characters. The centrosome is an organ 

 which is characteristic chiefly of the cells of animals: in the great majority 

 of these cells it has been found, at least during certain stages. In plants 

 centrosomes are limited to the cells of algae and fungi and the spermato- 

 genous cells of certain bryophytes and pteridophytes. If the blepharo- 

 plast be regarded as a modified centrosome, a question which will be 

 discussed further on, all motile cells (spermatozoids) of bryophytes, 

 pteridophytes, and gymnosperms must be looked upon as possessing 

 centrosomes. During the last decade of the nineteenth century several 

 botanists reported the presence of centrosomes in the cells of a number of 

 angiosperms, but these cases have all failed to stand the test of subsequent 

 more critical research. 1 



It is scarcely possible to give a description which will apply to all 

 centrosomes, since to any rule there are apparently exceptions. The 

 "typical" centrosome, as seen in animal cells, is a very minute granule, 

 which stains intensely with certain dyes. It is usually situated in the 

 cytoplasm, but in some cases it is found within the nucleus (Fig. 18). 

 It commonly lies in a more or less hyaline mass of material, called the 

 centrosphere (Strasburger 1892), attraction sphere (van Beneden 1883), 

 astrosphere (Fol 1891), or hyaloplasm sphere (Wilson 1901). 2 This centro- 

 sphere may often show two or more concentric zones differing somewhat 

 in structure and appearance (Fig. 60). At certain stages, especially 

 during nuclear division, the centrosome becomes the focus of a system of 

 delicate rays known collectively as the aster (Fol 1877). The aster will 



1 For a review of these cases see Koernicke (1903, 1906). 



2 There has been much confusion in the application these terms. (See Wilson 

 1900, p. 324.) 



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