6 
Davis . — Spore Formation in Derbesia. 
protoplasm. Segmentation takes place by cleavage furrows which start 
at the periphery of the protoplast within the sporangium and cut into the 
protoplasm in the form of curved and branching furrows (Fig. 12). 
The cleavage furrows first mark out fairly large areas which may con- 
tain a number of nuclei, but the areas become successively smaller as new 
furrows are formed at the periphery or strike off from the sides of the older 
into the protoplasm. Finally, the furrows so divide the protoplasm that it 
becomes blocked out into approximately equal, uninucleate masses (Fig. 13), 
and these gradually round up as the zoospore originates. The zoospores 
are almost invariably uninucleate, but I have found binucleate examples 
(PL II, Fig. 27) with blepharoplasts fully developed. These undoubtedly 
result from conditions similar to the formation of bi- and tri-nucleate eggs 
of Saprolegnia and some species of Albugo (Davis, ’ 03 , pp. 243 and 324), 
i. e. two or more nuclei have been able to exist sufficiently near one another 
to be included in the same area of protoplasm when the reproductive cells 
were formed. 
The plastids in the zoospore-origins and young zoospores (Fig. 14) are 
arranged about the nuclei with their longer axis radiating outward, so that 
the nucleus has very conspicuously the appearance of being the centre 
of the cell not only geometrically but also dynamically, as shown by the 
finer structure of the protoplasm. The radiating strands from the cyto- 
plasm investing the nucleus are prominent, each with a granule as its 
base. The protoplasmic strands run out into the cytoplasm between the 
plastids, but the latter are so densely crowded that it is impossible to trace 
the strands far, and I was not able to determine whether they reach the 
periphery of the zoospore and thus form a radiating mesh between the 
nucleus and the outer plasma membrane. It seems probable, however, 
that they do so. 
The Development of the Blepharoplast and its Cilia. 
The blepharoplast of the zoospore of Derbesia is at maturity a re- 
markable structure. It consists of a double ring (Fig. 22) situated just 
underneath the plasma membrane. The cilia are developed from the 
lowermost of the two rings and radiate out so as to form a circle or crown 
at one end of the zoospore (Fig. 1). The blepharoplast is generally about 
one and a half times or twice the diameter of the nucleus. The study 
of such a large and remarkably shaped structure presented exceptional 
opportunities for the investigation of some disputed points concerning 
the origin and relation of this cilia-forming organ to the nucleus and the 
outer plasma membrane of the cell. 
Previous to the formation of the blepharoplast the nucleus moves from 
near the centre of the zoospore-origin towards the periphery. It is then 
clear that the radiating strands on the side of the nucleus nearest the 
