78 HELEN DEAN KING. 



spindle (Fig. 4, CM). Isolated masses of chromatin are some- 

 times found near the spindle at a much later period when the 

 chromosomes are at the equator preparing to divide. They have 

 entirely disappeared by the time the first polar body is given off, 

 possibly serving as food for the cytoplasm as suggested by Gar- 

 diner (7). 



During the next half-hour, the irregular chromatin masses 

 change into chromosomes with a definite shape. The change 

 does not take place at the same time in all of the chromatin 

 clumps ; in fact, until the chromosomes are arranged at the equa- 

 tor of the spindle ready to divide, they may be found in several 

 different stages of development on the same spindle. Twelve 

 chromosomes, one-half the number characteristic of the somatic 

 cells of this species differentiate from the chromatin masses. The 

 chromosomes are scattered over the entire spindle and are at first 

 somewhat triangular in shape (Fig. 3), later they become rod- 

 shaped structures which may lie with their long axis parallel, 

 oblique, or even at right angles to the longitudinal axis of the 

 spindle (Fig. 4). Sooner or later, however, the long axis of each 

 chromosome comes to lie parallel with the spindle fibers and the 

 chromosomes then have a rounded knob in the middle region 



o 



and frequently also a smaller knob at each end (Figs. 4, 5). 

 Later the middle knob becomes more prominent and the end 

 knobs disappear (Fig. 5). 



At the stage of Figs. 2-3 the asters at the spindle poles reach 

 their greatest development. There are many long rays from 

 each aster which run nearly parallel with the spindle fibers and 

 cross each other at the equator of the spindle, and fewer and 

 much shorter rays going out in other directions. Soon after this 

 time the asters begin to degenerate. The shorter rays disappear 

 first and by the time the spindle has reached the periphery of the 

 egg there is not a trace of the radiation left. The spindle fibers 

 then converge at the poles which are surrounded by a small ac- 

 cumulation of granular substance probably formed from the dis- 

 integrated rays (Fig. 7). 



There is often a marked difference in the size of the chromo- 

 somes on the same spindle even when they are of exactly the 

 same shape. One or two of the chromosomes may extend over 

 one-third the length of the spindle, the others being not more 



