344 Mottier . — The Development of the Heterotypic 
and rather regularly arranged. In it all traces of the longitudinal fission is 
concealed, save in exceptional cases. 
In Liliinn the arrangement of the spirem is less regular and less easily 
followed. The longitudinal fission is as a rule also obscured. 
The hollow or loose spirem is followed by a rearrangement of the 
thread which has been called the * second contraction ’. This second 
contraction consists in the arrangement of a large part of the spirem into 
loops that tend to radiate from a closely entangled central mass of the 
thread. The loops are formed by the proximation of the parallel portions 
of longer turns of the spirem. In Lilium , as the rearrangement into loops 
takes place, it frequently happens that the halves of the longitudinally split 
spirem tend to divaricate rather widely in places. Later, these divaricated 
portions come together again, and the fission is obscured. During the 
second contraction the cross segmentation begins. The free ends of 
the loops may lie near the centre with the head or bend towards the 
nuclear membrane, or the reverse may be true. The parallel sides of 
the loops are frequently closely applied and twisted about each other. 
Each loop represents a bivalent chromosome, each parallel part being 
a single chromosome. The two parallel parts, or chromosomes, were 
arranged tandem, or end-to-end, in the spirem. All of the bivalent 
chromosomes do not arise as loops ; the two parts or segments may 
approximate in various positions. This fact accounts for the variously 
shaped chromosomes observed in later stages. The more entangled parts 
of the spirem form the curiously curved and twisted bivalent chromosomes. 
The rearrangement or second contraction is a regularly occurring step in 
the heterotype mitosis in all the plants examined. 
As the bivalent chromosomes separate from the second entanglement, 
they tend to become distributed within the nucleus and frequently near 
the periphery. In Tradescantia and Galtonia , there is in many cells 
a marked tendency for some of the segments to adhere end-to-end, forming 
chain-like rows of sausage or kidney-shaped pieces. Whether this is due 
to incomplete cross segmentation, or to a re-adherence subsequent to cross 
segmentation, cannot be stated. Other chromosomes in the same nucleus 
may have the two parts so arranged as to form open or closed links, or 
these may lie side by side as two short thick pieces. In all the plants 
studied, the two members of each bivalent chromosome were not side by 
side in the spirem, representing the halves of the longitudinally split 
thread, but they were two different parts of the spirem that were arranged 
end-to-end in the chromatin thread. 
As the two members of the bivalent chromosome separate in meta- 
kinesis or during the anaphase, each is seen to be split longitudinally, and 
I am convinced that this is the original longitudinal split seen in the early 
prophase. This longitudinal split represents the line along which the 
