THE CENTROSOME AND THE BLEPHAROPLAST 



79 



at least for a number of cell generations. In other cases it disappears at 

 the close of mitosis, a new one being apparently formed just before the 

 next mitosis. The fact that the formation of centrosomes may be 

 brought about by artificial means suggests that the regular appearance of 

 the centrosome in successive mitoses is closely associated with regularly 

 recurring physiological conditions in the cell; and that its presence in 

 successive cell-divisions does not require an uninterrupted morphological 

 continuity through the intervening stages. Its constant presence in some 

 tissues probably indicates the continuity of some physiological function. 

 Centrosomes in Algae. 1 One of the earliest known centrosomes in 

 plants was that of the diatom Surirella, discovered by Smith (1886-7) 

 and Blitschli, and fully described by Lauterborn (1896) and Karsten 

 (1900). It lies near the nucleus, becomes surrounded by radiations, and 

 divides to form the central spindle of the mitotic figure in a very 

 peculiar manner. 





FIG. 21. Centrosomes in algse. 

 A, Stypocaulon. (After Swingle, 1897.) B, Stypocaulon. 



(After Escoyez, 1909.) C, 



eentrosphere-like bodies in Polysiphonia. 

 dichotoma. (After Mottier, 1900.)- 



(After Yamanouchi, 1906.) D, E, Dictyota 



Centrosomes in the Sphacelariaceae have been described by Humphrey 

 (1894), Swingle (1897), Strasburger (1900), and Escoyez (1909). In the 

 vegetative cells of Sphacelaria, according to Strasburger, the centrosome 

 is situated in a centrosphere at the focus of an aster. Previous to mitosis 

 it divides into two which take up positions at opposite poles of the 

 spindle. In Stypocaulon (Swingle) essentially the same condition exists 

 (Fig. 21). Escoyez later concluded, however, that the asters of Stypocau- 

 lon are formed independently rather than by division, and that the 

 central corpusclse are probably not true centrosomes, but cytoplasmic 

 microsomes. 



1 This review of plant centrosomes and also that of the blepharoplast in subsequent 

 pages are based upon similar reviews given by the author in his paper on Spermato- 

 genesis in Equisetum (1912). 



