82 H. B. GOODRICH 



B. Discussion 



From the foregoing description it is apparent that the motion 

 of the X-group to one pole is determined by its attachment to 

 one half of a bivalent unit (the Y and the end of the long X- 

 component) that divides equally and the X-elements follow 

 or are dragged by that part with which they are united. Other- 

 wise this group is subject to the same forces, be they repulsion 

 or attraction or pull from the centers^ as are the other chromo- 

 somes. During the first phase the seven smaller X-chromosomes 

 are held rigidly perpendicular to the spindle axis as if in a state 

 of tense equilibrium under the influence of equal and opposing 

 forces. As they are non-separable units they remain in this 

 condition in the center of the spindle, not dividing and moving 

 apart like the halves of the bivalent autosomes. During the 

 second phase these forces from the centers do not seem to oper- 

 ate; evidence of tension is gone, fibers become bent, granulated 

 and disappear and the X-group moves from its position as if 

 dragged by the autosome plate to which it is attached. It 

 is true that sometimes straight fibers maybe observed attached 

 to the X-group on that side toward which it is to move and 

 bent fibers on the opposite side (fig. 40). It is as if the fibers 

 were pushing and pulling but such forces would tend to resist 

 the motion of the X-group to its position parallel to the spindle 

 axis and it seems better to regard the conditions as due to the 

 effect that this movement would have upon attached but in- 

 active fibers. It is not impossible that the movement of un- 

 mated X-chromosomes may in some cases be due to an unob- 

 served attachment to a bi^'alent group. The description by 

 Kornhauser ('14) of the first maturation division in Enchenopa 

 curvata is suggestive of such conditions. 



^ The shortening of the spindle as mentioned in the description woukl indi- 

 cate an attractive force. It is possible that the measurements (all that could 

 be obtained from one testis) are not sufficiently numerous to be significant for 

 such a slight difference. Meek ('15) however has found in Forficula that the 

 spindle is often shorter in late metaphase and early anaphase stages than in 

 early metaphase stages. 



