164 CANNON: STUDIES IN PLANT HYBRIDS 
which stray into the cytoplasm from the two poles of the 
spindle. 
My material was not suitable for the study of the early history 
of the chromatin of these cells and therefore the account of the 
maturation divisions begins with the spore mother-cell and in- 
cludes the two maturation mitoses by which the tetrads are 
organized. ; 
The first indication of the approaching first division of the 
nucleus of the spore mother-cell which was observed was the 
aggregation of the chromatin to one side of the nucleus and the 
formation there of the spireme. Thus synapsis occurs before the 
mother-cell withdraws from the cell-wall (#7 7). The spireme 
ribbon loosens and at length its convolutions extend throughout 
the nucleus, At first the spireme has a beaded appearance and 
an uneven outline, but in the older nuclei the contour is much 
more uniform and the structure much more dense. At the same 
time the spireme increases in its power of absorbing stains. Par- 
ticularly in the later stages the splitting of the spireme was 
observed with great clearness. 
When the spireme forms segments, the chromatin for a second 
time collects to one side of the nucleus, and in that situation the 
shortening and thickening of the segments takes place. The.seg- 
ments are long and in delicate loops when first clearly made out; 
the loops are much twisted and bent in a variety of ways. The 
ends of the loops are not infrequently connected by delicate proto- 
plasmic strands, indicating perhaps that the fusion of the adjacent 
chromosomes had not been a complete one (f. 4), and it also 
may happen that the union of the chromosomes did not take place 
exactly at their ends but at a greater or less distance from them. 
Usually, however, the loops are entire and symmetrical and give 
no hint as to their possible origin or formation. Very frequently 
in the long chromatin loops a longitudinal splitting may be seen, 
which must evidently be regarded as a precocious process looking 
to a subsequent separation of the divided portions. 
The chromatin loops shorten and thicken to form rings and 
they in turn by further condensation lose the ring character, but 
in the metaphase of the first division this character reappears. 
In any nucleus the loops and the rings are of a uniform size, 
