1905] MOTTIER—HETEROTYPIC CHROMOSOMES 173 
into two pieces of equal length. Thus arise twelve chromosomes 
which come together in pairs to form the bivalent chromosomes. 
FARMER and Moore (’os) in their joint publication have pre- 
sented the results of their observations upon animals and plants. 
Of the latter the familiar and oft studied Liliwm candidum heads the 
list, and to this form alone reference will here be made. As has fre- 
quently been described, the chromatin ribbon of Lilium candidum 
splits longitudinally, and the halves usually separate more or less 
widely from each other. Later the halves reapproximate, and the 
split closes up again. At the same time the entire spirem shortens 
and thickens. The contraction goes on rapidly, and the original 
longitudinal split soon ceases to be noticeable, being visible in excep- 
tionally favorable cases only. A rearrangement of the thread now 
sets in, such that parts of the spirem become pulled into parallel 
positions. This is well seen in those places where, at the bend of a 
convolution, an attachment to the nuclear membrane has taken place. 
In this manner, a close and parallel approximation of lengths of the 
entire spirem is effected; and this parallel arrangement, it is stated, 
has been commonly interpreted as representing the parallel split 
halves of the spirem thread. As a consequence of this rearrange- 
ment of the spirem, or parts which give ‘rise to chromosomes, the 
segments when isolated very often exhibit the form of a loop, open at 
one end, with sides either parallel to each other, or more frequently © 
twisted over one another. All chromosomes are not formed in this 
way however. Sometimes two, more or less straight, rodlets may 
unite so as to give rise to figures of rings, ellipses, etc. The point 
especially emphasized by the joint authors is “that the two rods, 
sides of loops, or whatever other form the structure as a whole may 
assume, represent, not the longitudinal halves of a split thread, but 
the approximation of serially distinct regions of the spirem as a whole. 
Thus each heterotypic chromosome is a bivalent structure, and their 
‘Teduced number is due to the approximation and mo less int 
Mate, though temporary, union of the equivalents of pairs of somatic 
chromosomes.” 
__ ATLEN (’os), who has made a very detailed study of this mitosis 
= Lilium canadense, does not conclude in favor of a reducing divi- 
Sion. He interprets his results as indicating that a longitudinal 
