Oct., 1923] 
DARLING — CHROMOSOME BEHAVIOR IN ACER 
453 
all joined together into a single long, tangled one, but after an examination 
of numerous cells of this stage, such a conclusion seems hardly warranted. 
These thickened threads soon become less tangled, usually with one end 
extending into a clump at one side of the nucleolus (PI. XXXII, fig. 13); 
this stage resembles that figured by some workers as the second contraction 
stage. Commonly one or more individual threads do not extend into the 
clump. Many of the threads at about this stage show a lighter longitudinal 
area indicating a longitudinal splitting; the splitting becomes so nearly 
complete in some cases as to separate the two halves of the thread for at 
least part of its length. These threads often contain as many as four or five 
deeper-staining places somewhat varied in size; on the split threads these 
appear as paired, equal-sized bodies (fig. 13). At about this stage the 
cytoplasm becomes drawn away from the cell wall and more condensed 
about the nucleus. 
The next apparent change in the cell is in the general distribution of the 
threads throughout the nucleus; many of the threads are bent or looped 
(fig. 14). As development proceeds, the threads become shorter, usually 
more ragged, and several of them soon become shaped like a C, a U, or an 0 ; 
at this stage the split has disappeared (figs. 15-17). One cannot be certain 
that each one of the threads folds over on itself; in fact, after studying 
numerous nuclei of this stage and after considering the subsequent stages 
in the separation of the individuals of a pair, I am of the opinion that some 
threads do fold over and some do not. There is no direct evidence that the 
bend in the thread occurs where the two chromatin bodies of a pair unite, 
but it is reasonable to suppose that such is the case. These bent threads 
become shorter, and some of them at least segment at the bend; contraction 
and condensation of the chromosomes continue while the membrane of the 
nucleus disappears (figs. 18, 19). Some of the threads become condensed 
into chromosomes before others; this fact may be associated possibly with 
the difference in size of the chromosomes which one observes at metaphase. 
This irregularity, or variation, in development is also evident as the chromo¬ 
somes are separated at metaphase, some individuals of a pair being entirely 
separated while others are intact. As the members of a pair are being 
separated, some of the elongated ones show rather clearly that they are 
lying side by side in the spindle. There is no evidence of splitting of the 
individual members of the bivalents during these stages. When the nuclear 
membrane disappears, the chromosomes become collected about the 
nucleolus, which in turn becomes elongated and irregular, gradually dis¬ 
appearing as the chromosomes become arranged on the spindle. 
As the univalent chromosomes move toward the poles they appear to 
become more condensed, forming short, thick lumps; there is some indica¬ 
tion of a splitting of the univalents during this stage, but the evidence is 
not convincing. At the poles the chromosomes retain their individuality; 
they become somewhat larger, irregular in shape, and some, at least, have 
