Baird—History of Phycornyces Nitens (Agardh) Kunze. 371 
Another minor variation in the process of nuclear division, oc¬ 
curring in Rhizopus as compared with Phycomyces, is that in 
Rhizopus the chromatin mass is separated more frequently into 
two portions rather than into three, four, or more as in Phycomyces 
(figs. 10a, 10b, 10c). In a few cases observed, however, in Rhizopus 
the chromatin may be separated into several portions (figs. 13a 
and 13b). 
Figures 11a and 11c illustrate cases in each of which the vacuolar 
membrane has progressed further than the two daughter chromatin 
bodies, thus leaving them within the vacuole, and figure 13b shows 
one such chromatin body forming a second vacuole within itself. 
Figures 13a, 13b, and 13c represent nuclei from a zygospore at a 
time when the associated vacuoles are becoming filled with homo¬ 
geneous reserve substance, as was described for the nuclear vacuoles 
of Phycomyces. 
Figure 12 represents a spore from the sporange of Rhizopus be¬ 
fore the spores have shrunk and their walls have thickened. The 
nuclear structures observed here apparently shrink as the spore 
matures and shrinks. Each structure thus becomes one of the 
dense nuclei observed in the mature spore and that appear as soon 
as the spore germinates, as was figured for Phycomyces (fig. 1). 
Nuclei of Aged Mycelium 
The nuclei of the vegetative, aged mycelium behave very simi¬ 
larly to those in the maturing zygospores. Nuclear division con¬ 
tinues in the vegetative hyphae up to a time which I conclude is 
determined by a change in water balance. For, at a time when no 
further growth of mycelium occurs, its nuclear vacuoles take on 
the appearance of those which contain reserve food in the zygo¬ 
spores (figs. 14a and 14b). In figure 14a, taken from a 36-day-old 
culture of Phycomyces, the vacuole has become filled with a homo¬ 
geneously-staining substance, and some of the nuclei associated 
with the vacuole are still partially differentiated. Figure 14b rep¬ 
resents a similar condition of a nuclear structure taken from a 
vegetative hypha of a 14-day-old culture of Rhizopus. 
As the mycelium becomes dried out in aging cultures, the slimy 
cytoplasm does not disappear from the mycelium on a large scale 
as it does in the zygospores. However, in the aged mycelium par¬ 
tially dried out, some of the hyphae appear empty, except for a 
few of the deep-staining nuclear structures and a very slight 
