II. The Cytology of the Gametophyte Generation. 197 
order to get a sufficient number of the ova large pieces of the plants 
have to be placed in the culture dishes. When such siliceous particles 
are present they often interfere with the microtoming, and cause the 
tearing of the ribbons and the consequent loss of sections. Even when 
the cutting has been successful there are so many eggs in each slice 
that the identification of the successive sections of any particular one 
is often a matter of some difficulty. To obviate this trouble it is often 
a convenient practice to include in the material some other easily cut, 
and sufficiently distinctive, object to serve as ‘landmark’ so to speak, 
by reference to which to locate the others. 
There are two ways in which an unfertilized egg-nucleus of Dictyota 
may initiate its division. In both cases the single nucleolus increases 
in size, and shows a tendency to break up into chromosome-like masses. 
In the mode shown in Fig. 30 the nuclear membrane disappears, though 
the limits of the nuclear cavity are still recognizable. A globular mass 
is found in the centre, which consists probably of both nucleoplasm and 
nucleolar substance. This is the least common method, but a fair number 
of examples has been met with. Fig. 31 shows a far more frequent 
case ; here the membrane is still intact, but the nucleolus seems to be 
fragmenting. The considerations that incline me to the conclusion that 
the nucleolus breaks up directly into chromosomes are the following : 
(a) In many hundreds of cases examined I have never yet seen 
a spirem. 
(b) As soon as the chromosomes are formed the nucleolus com- 
pletely disappears ; neither nucleolar globule nor chromatin fibrillae 
remain. 
While the change in the nucleolus is going on the nuclear network 
preserves its usual appearance. Outside the nucleus there is nothing 
suggestive of either centrosome or polar radiation, while the fertilized egg- 
nucleus on the other hand has a distinct centrosphere (Fig. 31). The 
cytoplasm and chloroplasts frequently retain the faintly radiate structure 
shown in Fig. 19. The stage where the chromosomes have been differen- 
tiated, and the nucleolus has very nearly disappeared, is seen in Figs. 30 
and 33. Not only is this stage distinguishable by the absence of nucleoli, 
but the chromosomes are only sixteen in number, and they are different 
from those of fertilized eggs in being thinner and less regular in form. 
Fig. 33 shows the only case observed where any fibres had appeared 
before the dissolution of the nuclear membrane. It is not unlike Fig. 34, 
but the very rudimentary appearance of the figure, and the fact that there 
are only sixteen chromosomes in the three sections of the figure, prove 
that it is merely an exceptionally regular case of parthenogenesis. 
It is a very striking fact that while in nearly all the normal mitoses 
described in these two papers the nuclear membrane persists until a very 
