STUDY OF CELL MECHANICS 483 



ber of theories have been advanced to account for the chromo- 

 some movement. Of these, three may be considered the more 

 important. (1) Contractibility of fibrillae. Klein (79), Van 

 Beneden ('83), Boveri ('88), etc. (2) A pushing apart of the 

 chromosomes; Growth of central spindle, Dri'mer ('95) etc., or 

 Watase's ('93) ingenious suggestion. (3) Possibly carried along 

 by protoplasmic movements, Butschli ('76), or by chemotaxis 

 Strasburger ('93). Most of the later writers on the subject have 

 been inclined to Biitschli's view. Wilson ('95), Griffith ('99), 

 Conklin ('02 and '05). 9 



Of the many objections to the contractile hypothesis, that 

 urged by Wilson, and by Griffith seems the most cogent. In 

 sea urchin eggs after the chromosomes have come to lie against 

 the edge of the centrosphere, they spread out over its surface, 

 a fact irreconcilable with the contraction theory. In monaster 

 eggs this spreading of the chromosomes is seen exceptionally 

 well. 



Any hypothesis which attributes the movement of the chro- 

 mosomes to interzonal fibers, can not explain the monaster 

 eggs, since no such structures are present. Yet the chromo- 

 somes approach the centrosphere with rapidity and certainty. 



This study of monaster eggs has brought to light no new facts 

 inconsistent with the view that the chromosomes are carried 

 to the aster by the flow of protoplasm. For as the chromosomes 

 divide, the aster begins a rapid increase in size. It is, interest- 

 ing to note that no matter how long the chromosomes remain 

 undivided, no great increase of the aster occurs until they are 

 cleaving. There is a close connection here between the two 

 phenomena. 



9 The question of chromosome movement, which was so much discussed in 

 the nineties, has recently received little attention. For a full review of the 

 literature of this earlier period see Wilson's 'The Cell,' pp. 70-77, also Conklin's 

 Karyokinesis and Cytokinesis. Although written some twenty years ago, Wil- 

 son's remarks still seem pertinent: "In any case, I believe that no satisfactory 

 hypothesis can be framed that does not reckon with the chemical and ph} r sical 

 changes going on in the centrosphere, and take into account also the probability 

 of a dynamic action radiating from it into the surrounding structures." ('The 

 Cell' page 77.) Such a theory has yet to be advanced, but several attempts 

 have been made in this direction. See Lillie's various studies on Cell Division. 

 Also Hartog ('14). 



