70 
Smith.—On Direct Nuclear Divisions in the 
marked. The parts of the nucleus farther away from the constrictions remain 
about the normal width, but the centre becomes gradually thinner until the 
two portions of the chromoblast are finally joined only by a thread (Fig. 9), 
at which stage the outer membrane appears hour-glass-shaped. For a 
short time the remnants of the thread joining the two daughter chromoblasts 
remain, but gradually they are withdrawn into the new bodies, which round 
off into the typical spherical form (Figs. 10 and 11). The constriction of 
the membrane becomes more acute until the two sides touch. Finally, the 
pull of the cytoplasm appears to draw the two new nuclei apart. The linin 
threads are irregularly distributed at the two ends, where they seem to hold 
the two daughter chromoblasts to the rapidly elongating membrane. 
The chromatin on the membrane does not seem to increase in quantity 
until the final stages, so that when the nuclear membrane first elongates 
these chromatin masses become stretched and therefore thinner. It is only 
after the daughter nuclei start to round off again that these slowly regain 
their original density (Fig. 11). 
The time at which nuclear division takes place is fairly constant, 
though there is reason to believe that a few divisions take place at all 
times of the day and night. From the results obtained from a series of 
preparations which were fixed at every thirty minutes throughout the 
twenty-four hours, it can be stated that the majority of the vegetative 
nuclear divisions take place between the hours of 10 p.m. and 2 a.m., 
a maximum being reached at midnight. All the nuclei in a hypha do 
not divide at the same time, and in any case are not all found in the 
same stages as is often the case in oogonia. 
This division about the time of midnight, together with other observa¬ 
tions made, indicate that, in spite of many exceptions, there is a regular cycle 
of events, causing the zoospores and sporangia to be found in a similar con¬ 
dition at a given time of the day. A brief outline of this cycle is given 
below, in which a variation of two hours either way may be allowed on the 
times given (see opposite page). 
Observations were made on living material, but in the hyphae them¬ 
selves no results were obtained, owing to the difficulty of seeing the nuclei. 
Even methylene blue was of no assistance, since it did not stain evenly, and 
the cytoplasmic granules absorbed most of the stain. In the zoospore 
better results were obtained, and in a germinating specimen a number 
of divisions were seen during the night. No nuclear structure was visible, 
but constrictions in the nuclei were observed, and when the material was 
stained with iodine some eight or nine nuclei appeared. The cytoplasm 
streamed all the time, and with it the nuclei moved rapidly. The germ- 
tube had a dense mass of cytoplasm in the tip, but the nuclei remained in 
the zoospore for some time. When branching was about to take place in the 
young hypha, a nucleus was found in the neighbourhood of the branch origin. 
