CYTOKINESIS. 



105 



movements of the sphere, the cell wall being especially prominent over the sphere. 

 He also discusses in a verj- suggestive manner the relation of these movements to 

 the morphological and physiological polarity of the cell. 



This is doubtless an incomplete list of the cases which have heretofore been 

 observed in which there is a decided bending of the spindle axis at the close of 

 division, but the cases are sufficiently numerous to indicate that this is probably a 

 general phenomenon. 



3. Orientation of Centrosonies and Spijidles. — The centrosome, which during 

 the anaphase is usually spherical, becomes ellipsoidal or spindle-shaped during the 

 telophase and rest. The axis of elongation of the centrosome becomes the initial 

 spindle axis. It is nearly constant in direction for any given cell generation, but 

 differs somewhat in different generations. No general rule can be formulated with 

 regard to the relation of this initial spindle axis to the old spindle axis, or rather 

 the half of it which lies in each daughter cell, but the new axis is most frequently 

 at right angles to the old, figs. 82, 83, 86, 91. 



As the initial spindle elongates and the peripheral layer of the old centrosome 

 disappears, the new spindle moves out of the old sphere, which at once becomes 

 irregular in outline ; at the same time the new spindle moves over the surface of the 

 nucleus until it comes to lie in the groove separating the germ halves and until the 

 poles of the spindle lie at opposite sides of the nucleus. In some cases the initial 

 spindle lies almost in the groove between the germ halves of the nucleus w^hen first 

 formed [e. g., prophase of the second cleavage, figs. 82, 83, text figs. VII, VIII) ; 

 in other cases it must move some distance before taking up this position [e.g., 

 earliest stages in the third and later cleavages, figs. 86, 88, 91). 



Fig. XXXII. Fig. XXXIII. 



Figs. XXXII, XXXIII.— Metaphase of the second aud third cleavages of Crepidula, showing an unusual position 

 of certain spindles; these ultimately rotate into normal positions. 



The position of the initial spindle in the nuclear groove does not always corre- 

 spond to its definitive position. In many cases the latter is reached only after more 

 or less extensive movements of the entire mitotic figure {c/. fig. 88 and text figs. 

 XXXII and XXXIII). These movements are of two kinds, a rotation of the spin- 

 dle into its definitive axis and a transportation of the entire figure to its final posi- 

 tion in the cell, figs. 93, 94. 



14 JOUEN. A. N. S. PHILA., VOL. Xll. 



