Nuclear Divisions in the Rusts. 
343 
which fused in the teleutospore, a short time before this stage. The great 
theoretic importance of the latter theory is sufficiently obvious, and will be 
discussed to a limited extent later in the general part of this paper. No 
signs of the doubling of the centrosome are to be noticed in the newly 
reconstructed spermatium nucleus drawn in Fig. 7, nor in Fig. 8, which 
represents a highly-magnified sporophytic nucleus from the young aecidium 
of Puccinia Cirsii-lanceolati. So far as can be seen, the centres in each of 
these figures are single structures. It is at once obvious, particularly in 
Fig. 8, that the chromatin is centred on this structure. One end of each of 
the chromatic filaments is connected with the centre, while the other 
extends into the nuclear cavity, thus resulting in a polarized condition 
of the nucleus, similar to that described for certain mildews by Harper (’ 05 ). 
Fig. 9 is from the spermogonium of Uromyces Scirpi , and represents 
an equatorial-plate stage. The ill-defined chromatin is massed somewhat 
excentrically in the equatorial region of the dense spindle. Centrosomes 
are somewhat obscure in this instance ; possibly the broad structure at the 
lower pole is to be regarded as a sort of pole-plate, or disk-shaped centro- 
some. 
The next three figures show gametophytic nuclei, as is true also of all 
the figures of nuclei so far described, except Fig. 8. Figs. 1-7 and 9, 
however, are all drawn from spermogonia ; while Figs. 10, 10 <2, and 11 are 
from the vegetative hyphae which finally produce the gametes. All three 
of the latter drawings show the anaphases of the division, by means 
of which the so-called ‘ sterile cell’ is cut off. Figs. 10 and 10 a are from 
Triphragmium , and Fig. 11 is from one of the hyphae from the base of the 
aecidium-cup of Uromyces Scirpi. In Fig. 10, reconstruction is in process. 
Above, the centre is obviously double, and about four strands are attached 
to each half-centre, just as was described for Fig. 6 a. The daughter- 
nucleus below, in Fig. 10, shows an obliquely polar view, so that about eight 
chromatic filaments can be counted, radiating from the pole. A new 
nucleole is in process of formation within this latter nucleus. In Fig. 10 a 
the mitotic figure is apparently rather poorly defined, as is also the late 
anaphase shown in Fig. ij. 
Figs. 12-22 show a fairly complete series of stages of the mitoses which 
take place in the sporophytic cells containing the so-called ‘conjugate 
nuclei \ While considerable growth takes place in the gametophytic hyphae 
which have just been considered, a greatly increased stimulus to growth is 
apparent at once on the initiation of the conjugate, or binucleated, con- 
dition, through the fusion of two cells. The first step in the rejuvenating 
sexual process is thus the sudden transition from uninucleated cells to 
binucleated. 
As has been described above, when the cytoplasm of two cells begins 
to fuse, through the gradual absorption of the wall which separates them, 
