certain Penaeaceae. 
365 
with the object of securing these divisions, and all the stages here figured, 
and also the dividing endosperm nucleus of Fig. 25, were found in material 
fixed on several occasions within a few minutes of mid-day. Other nuclear 
divisions showed no such relation to any particular time of day. The meta- 
phase of the heterotype division was only seen in one case (Fig. 6), the 
spindle being here arranged obliquely to the axis of the ovule. Thegame- 
tophytic number of chromosomes is eleven or twelve. The metaphase of the 
homotype division is shown in Fig. 7, and the four daughter-nuclei derived 
from this division in Figs. 8-12. These nuclei usually lie tetrahedrally or 
crosswise, but they show some variation in this respect ; thus in Fig. 10 three 
are grouped at the bottom of the sac, and the fourth lies towards the top. 
As the embryo-sac enlarges a central vacuole is formed, and these 
nuclei are relegated to the parietal layer of protoplasm (Figs. 11, 12). 
Each nucleus now divides (Fig. 12), so that four pairs of nuclei are formed 
(Fig. 13). In one case five groups of nuclei, consisting of four pairs and one 
single nucleus, were present at this stage (Fig. 14). Possibly this may mean 
that one of the four original nuclei has divided in advance of the others, 
and its daughter-nuclei have divided again after becoming separated as 
the sac enlarged. This preparation was fixed in acetic-alcohol, and is so 
much contracted that the wall of the embryo-sac has become drawn in, and 
the nuclei of the nucellar cells which have been crushed by the growth of 
the sac can be seen imbedded in it. This was the only case observed, at 
this or at any later stage, where more than the normal number of nuclear or 
cell groups were present. The four pairs of nuclei normally formed are 
found usually lying crosswise, one at each end of the sac, and the two at 
the sides ; and as the embryo-sac shows much less variation in the arrange- 
ment of its nuclei at this than at the four-nucleate stage, it is probable that 
the nuclei usually take up this position as the sac elongates. This also is 
the arrangement most commonly found in the mature embryo-sac (cf. Figs. 
17, 18, 25, 26), but it should be noted that both the free-nuclear groups, 
and the cells that are later formed from them, may take up any position 
around the periphery of the sac (cf. Figs. 15, 16, 19, 21). 
Each of the nuclei composing these four pairs now divides again 
(Fig. 15), so that four groups, each consisting of four nuclei, are formed 
(Fig. 16), one group from each of the four original nuclei. The two successive 
divisions to form these tetrads are usually simultaneous, but occasionally one 
nucleus divides in advance of the others. Protoplasm now aggregates around 
three of the nuclei in each group, and a definite limiting membrane appears 
round each of the cells thus formed, 1 while the fourth nucleus remains free 
(Fig. 17). While these membranes are appearing, or even after they have 
been formed, the fourth free nucleus of each group migrates to the centre of 
the embryo-sac (Fig. 18). There these four meet (Fig. 19), and gradually 
1 Exceptionally, two nuclei may be enclosed in a single cell (cf. Fig. 25). 
