THE ACHROMATIC FIGURE 



159 



rays actually fuse laterally to form the membrane, an interpretation 

 which has been disputed by Faull and others. 



Very similar to the amphiastral figures just described are those in 

 certain algse (Fig. 89). In Dictyota, for example, a curved rod-shaped 

 centrosome lying against the nuclear membrane divides as mitosis begins. 

 The daughter centrosomes, each with an aster, separate and occupy the 

 poles of the intranuclear spindle. In Fucus the two centrosomes are 

 said to arise independently. In Polysiphonia the centriole-like bodies 

 seen in the prophase give way later to larger centrosphere-like masses. 

 In living cells of Sphacelaria, according to W. Zimmermann (1923), the 

 conspicuous rays about the centrioles before the metaphase are made up 



Fig. 89. — Centrosomes in algse. A, B, Stypocaulon. (After W. T. Swingle, 1897, 

 and Escoyez, 1909.) C, centrosphere-like bodies in Polysiphonia. {After Yamanouchi, 

 1906.) D, E, Dictyota dichotoma. {After Mottier, 1900.) 



of the oriented boundaries of large vacuoles, with fucosan globules and 

 chromatophores lying along them. 



Amphiastral figures in the spermatogenous cells of bryophytes and 

 those vascular plants having motile male gametes are described in Chapter 

 XIV (Fig. 122). 



The Mechanism of Mitosis. — It has always been tempting to specu- 

 late upon the mechanical factors involved in the remarkable process of 

 mitosis. Although the problem is still very far from solution, it is of 

 interest to consider briefly some of the suggestions which have been 

 made." 



One of the simplest and most widely accepted theories was that of 

 fibrillar contractility proposed by Klein (1878) and van Beneden (1883, 

 1887), according to which the chromosomes were supposed to be dragged 

 apart by the contraction of two opposed groups of spindle fibers. Many 

 observations were cited in its favor, and elastic models were made to illus- 



^1 See the reviews by Wilson (1900, 1925) and Tischler (1921-1922). 



