No. 463.] STUDIES OF PLANT CELL— VI. | 485 
dently out of the reticulum. The two threads gradually fuse so 
that in older stages of synapsis the nucleus appears to contain a 
single relatively thick spirem which is shorter and more loosely 
coiled than in the earlier stages. The minute structure of the 
threads of the spirem can be determined by careful staining. 
They consist of a series of chromatin granules (chromomeres) 
imbedded in the ground substance, linin. As the two threads 
fuse the chromomeres generally come together in pairs and unite 
to form a single row of large chromomeres which project from 
the side of the larger single (fusion) spirem. 
The single (fusion) spirem on emerging from synapsis becomes 
uniformly distributed throughout the nucleus. There appear to 
be no free ends in the much convoluted and looped thread. 
Some of the loops become fastened to the periphery of the 
nucleus but there is no regularity in the number of loops and no 
relation to the number of chromatic segments that are formed 
later. While thus evenly distributed the single spirem under- 
goes a longitudinal fission which is preceded by the division of 
each chromomere. This is the first longitudinal fission of the 
spirem which is well known through the descriptions of Guignard, 
Grégoire, Strasburger, Mottier, and others. The fission is not 
simultaneous throughout the length of the spirem, for some por- 
tions remain undivided for some time when contiguous parts are 
plainly split. The result is a condition very similar to that pre- 
sented just before the fusion of the two systems of threads 
during synapsis which produced the single (fusion) spirem. It 
seems probable that the threads which become separated are 
morphologically the same as those which fused during synapsis 
although the union at that period seems complete. The split 
spirem remains uniformly distributed throughout the nucleus 
exhibiting, however, a tendency to become somewhat massed in 
the center of the nuclear cavity leaving fewer loops attached to 
the nuclear membrane. | 
The split spirem now segments throughout its length into the 
reduced number of chromosomes (12) characteristic of the 
heterotypic mitosis. The segmentation is not simultaneous, but 
the first free ends appear near or at the periphery of the nucleus 
where the split spirem breaks apart at the loops. As segmen- 
