68 Vniversity of California Publications in Zoology [Vol. 20 



with the chromosomes still joined by an end to end union in the 

 equatorial plane. The behavior of the heteroehromosome is not always 

 easy to determine in this stage. In figure 56, plate 10, it is found 

 lying near one end of the mass of chromosomes. Its attachment to 

 distinct spindle fibers could not be demonstrated. In figure 59, 

 plate 10, it has moved nearer the equatorial plane but is still outside 

 the main mass of chromosomes. 



Metaphase: The equatorial plate is wide and usually heavily 

 stained, with the chromosomes closely massed together. It is appar- 

 entl.y of shorter duration than either the prophase or later stages. 

 With the elongation of the nucleus the chromosomes separate at the 

 middle point, i.e., at the apex of the looped thread or V of the chromo- 

 some before it became attached to the spindle (pi. 10, fig. 60; pi. 11, 

 fig. 62). 



During the later prophase the small chromatin thread or hetero- 

 ehromosome is often obscured, particularly in the formation of the 

 equatorial plate. "With the separation of the chromosomes in the 

 metaphase, however, this again becomes prominent. It is found that 

 the vesicle has disappeared and the single thread has divided (pi. 10, 

 fig. 61). The separation of the two new heterochromosomes in the 

 metaphase lags somewhat behind that of the other chromosomes, hence 

 these usually may be seen between the two groups as they pass towards 

 the poles (pi. 11, figs. 62, 64). The attachment of these chromosomes 

 to spindle fibers in these stages, as in the earlier ones, has not been 

 determined. 



Anaphase: The separation of the chromosomes after the final 

 parting seems to take place by reason of an elongation of the entire 

 nucleus in the eqiiatorial region, rather than by a shortening of the 

 spindle fibers, since the chromosomes have in no case been found 

 closely attached to the poles. The spindle fibers remain approxi- 

 mately the same length throughout the anaphase until they disappear 

 in the telophase (pi. 11, figs. 61-65, 73, 74). The elongation of the 

 nucleus in the equatorial region is accompanied by a constriction of 

 the nuclear membrane. It is usually drawn out into a long, slender 

 strand before the final break occurs which separates the two daughter 

 nuclei (pi. 11, figs. 71, 72). At the same time the paradesmose also 

 lengthens as the centroblepharoplast complexes of the newl.y forming 

 daughter cells move farther apart (pi. 11, figs. 68, 72, 75). 



The connecting nuclear thread thus formed soon breaks and the 

 elongated portion of each daughter nucleus is gradually withdrawn 



