Chromosomes in Pollen Mother-cells. 32 7 
this can be seen in exceptional cases. The shortening of the chromosomes 
continues, and they are soon more regularly spaced within the cavity 
(Fig. 33). There is no great regularity in the spacing ; sometimes they 
seem to be quite regularly disposed, but frequently two or more lie close 
together or in contact. Fig. 33 represents the nucleus just prior to the 
formation of the multipolar spindle complex, and the chromosomes have 
about the form that they will present in the equatorial plate apart from a 
further shortening. The chromosomes show clearly that some arose as loops, 
while others may have come about by the approximation of two pieces 
which may, or may not, have been entirely separated before the approxima- 
tion. The segments are generally twisted about each other, although this 
is not universally true. It is not necessary to mention the various shapes 
of the chromosomes as found after their formation from the spirem and 
upon the nuclear spindle, since the various forms arising from the manner in 
which the two pieces are oriented with respect to each other have already 
been figured in superfluous profusion by other observers. It may be 
stated, however, that the number of phantastically-shaped chromosomes 
will depend upon the degree in which the spirem is entangled, knotted, 
and kinked during the second contraction and earlier. It is reasonable 
to believe that chromosomes coming from such nuclei as Figs. 29 and 
31 will be less regular in form than those arising out of those similar 
to Fig. 30. 
When the formation of the loops during the second contraction is duly 
considered, and when it is recalled that the chromatin thread shortens 
comparatively little in the stage of the hollow spirem, and that the sister 
halves of the thread, if divergent, become again closely applied, — only one 
explanation seems now possible to the writer, namely r that the twisted 
parallel parts of the loops shown in Fig. 30 result first from a looping 
of the spirem, followed by a more intimate approximation of the parallel 
sides of the loop. The somatic chromosomes are, therefore, arranged end- 
to-end in the spirem and not parallel side by side. There seems to be 
no good ground for believing that the longitudinal halves of the spirem 
tend always to diverge widely for longer or shorter stretches and to remain 
thus separated ; that the divergence is followed by a sudden shortening 
and cross segmentation into chromosomes ; and that the daughter segments 
fuse at one end following cross segmentation to form the loops so frequently 
and so abundantly present. Such a process, or one similar, must be assumed 
by those who believe in the doctrine that the longitudinal fission is the 
lateral union of two spirems in synapsis, and that the two segments of each 
chromosome appearing in the equatorial plate of the heterotypic spindle 
separate along the line of longitudinal fission. 
Trade sc antia. The development of the heterotype chromosomes in 
Tradescantia virginica agrees in general with that described in the pre- 
B b 2 
