520 THEOPHILUS S. PAINTER 



the two division centers are being carried away from the posi- 

 tions which they occupied when the spindle was first formed. 

 In the Hght of the facts recorded above, the explanation of this 

 condition seems clear. We are dealing here, I believe, with a 

 case of delayed cleavage. The first two cleavage planes, in 

 Cerebratulus stand at right angles to each other, just as in the 

 case of the sea urchin. This egg was delayed for some reason 

 in its division, after the spindle was formed, and we see the asters 

 being carried by protoplasmic currents to the new plane where 

 the division of the protoplasm should occur. 



Among certain of the leeches, the spiral aster seems to be a 

 normal feature in the development of definite blastomeres and 

 in the polar body formation. lijima ('81) figures it in eggs be- 

 fore the polar bodies have been given off, but his description is 

 too meagre to allow us to draw any conclusions as to the cause. 

 Later than this, we have the works of Sukatschoff ('03) on the 

 development of Nephelis. This author figures spiral asters as a 

 constant feature in the development of certain blastomeres and 

 advances, tentatively, an explanation for this. After pointing 

 out that we frequently have a shifting of blastomeres during 

 development he suggests that the two cells where he has constantly 

 found the spiral asters rotate on each other. ^^Nehmen wir nun 

 ferner an, — was a priori nicht ausgeschlossen erscheint — , dass 

 die ausserste plasmatische Schicht der Zelle in sich eine grossere 

 Kohasion besitzt, als das innere Plasma, so miissten natiirlich 

 die Vorher gerade verlaufenden strahlen in der Richtun^ der 

 Drehung spirahg gedreht werden (p. 333, I.e.). 



This author admits, however, that his explanation will not 

 apply to those cases where the spiral asters appear during polar 

 body formation. 



A glance at the figures given by Sukatschoff shows that the 

 rays of the asters run in the same way that I have described for 

 the sea urchin, that is, forward for a part of their length and then 

 backwards. It is scarcely to be doubted that the cause of the 

 bent rays lies in the protoplasmic movement of certain parts of 

 the cell outside of the asters themselves. Since the turning is a 

 constant feature for certain blastomeres, it probably has some 



