654 Beer. — Studies in Spore Development \ III . 
diarch structure (Fig. 29), and this subsequently develops into a bipolar 
structure (Fig. 31). The bipolar spindle is remarkably well defined and is 
characterized by its very acute and attenuated pole ends. No centrosomes 
are to be observed at this or any other stage of the division. The chromo- 
somes become arranged upon the equator of this spindle. They are at this 
time smaller and denser bodies with a more even outline than they 
previously possessed. At this stage an estimate of their number can 
be more satisfactorily made than at any other time. Although I have 
made repeated counts I have found it impossible to do more than arrive at 
an approximate estimate of the chromosome number in Equisetum . Where 
the chromosomes are so numerous and closely crowded together it is almost 
inevitable that some should become hidden beneath their neighbours and 
thus be missed when the count is made. 
Moreover, when an equatorial plate of chromosomes appears in more 
than one section, as is usually the case, the same chromosome may be cut 
so as to be represented in two (or more) sections, and it then becomes 
extremely difficult to avoid counting it twice over. In those cases in which 
the clearest view of the chromosomes could be obtained by counts varied 
from 94 to 136. Probably the mean of these numbers, viz. 115, is not very 
far removed from the true number of chromosomes in Equisetum. 
The form of the chromosomes when arranged upon the equatorial 
plate of the spindle is shown in Fig. 30. The spindle fibres, which are 
attached to the chromosomes, are each composed of a bundle of fine 
filaments, as can be seen from the drawing (Fig. 30). During the metaphase 
the spindle poles show no trace of polar radiations, but during the anaphase 
when the chromosomes are approaching the poles radiations extending into 
the cytoplasm become apparent (PI. LI II, Figs. 32, 33). 
The distribution of the univalent chromosomes to the two poles takes 
place very irregularly. From Fig. 32, which represents a typical case, 
it will be seen that whilst some of the chromosomes are already far on their 
way to the spindle ends, others are as yet only commencing their journey. 
The majority of the chromosomes become more or less elongated in form 
during the earlier stages of the anaphase as though stretched longitudinally 
by the spindle fibres. On reaching the poles of the spindle the chromo- 
somes again become short, dense bodies which are small in comparison 
to their size at an earlier period (Fig. 33). They become so closely massed 
together at each pole that it is often difficult to distinguish the outline 
of the separate chromosomes. Nucleolar bodies, extruded into the cyto- 
plasm, can frequently be still recognized at this stage. 
Soon the mass of chromosomes begins to loosen again, and a new 
nuclear membrane develops round each group. As the chromosomes move 
apart their soft mucilaginous substance remains adherent at certain spots 
which become drawn out into connecting branches between the separate 
