Cytology of Pythium tiltimum , n. sp. 295 
of the oospore which sim'ultaneously takes place, we may 
with safety conclude that the whole process of digestion and 
absorption is the work of the young oospores themselves. 
It is well to note that no appreciable thickening of the 
oospore-membrane takes place until the last traces of peri- 
plasm have disappeared. The first stage in the maturation 
of the oospores is marked then by the digestion and absorp- 
tion of the periplasm by the growing oospores. 
The second stage, represented in Figs. 28 to 32, is marked 
by three distinct processes of differentiation, viz. the thick- 
ening of the spore membrane, the fusion of the male and 
female nuclei, and the development of the reserve globule. 
The primary membrane of the oospore remains thin and 
appears to become slightly cuticularized, staining deeply with 
gentian violet. New layers are, however, added on the inside 
at the expense of the protoplasm, until a very considerable 
thickness is reached. Whether we are justified in speaking 
of an exospore and endospore in this case is doubtful, for as 
we shall see, the very thick inner wall is essentially a mass 
of reserve material. The two nuclei approach each other — 
as indeed they do at an earlier stage, as may be seen by 
comparison of Figs. 25 and 28 — but the actual fusion is 
delayed for a considerable period. Apparently ripe oospores 
are sometimes found to contain two closely approximated 
nuclei, as shown in Fig. 32, but this is very exceptional. 
The ripe oospores are uninucleate, and uninucleate oospores 
may be found in cultures four days old. The actual fusion 
is not difficult to trace, as the nuclei at this stage are in the 
resting condition and fairly conspicuous objects. Three stages 
in fusion are represented in Figs. 29, 30, and 31. In the ripe 
oospores there is constantly to be found a globular mass 
which in the main gives the proteid reactions. This mass 
may have one large cavity in its interior, when its form 
becomes that of a hollow sphere ; or it may have numerous 
small cavities, in which case it possesses a spongy character. 
The mass corresponds to the reserve globule seen in the 
living material. It has generally been considered to be of 
