36 6 Stephens . — The Embryo-Sac and Embryo of 
fuse to form the primary endosperm nucleus. A series of stages of this fusion 
is shown in Fig. 20. The process of fusion seems to occupy a comparatively 
long time. The nuclei can usually be seen in contact in the centre of the sac 
in the ripe flower bud, but the mature definitive nucleus (Fig. 20 d) has 
only been seen in a couple of cases where some of the other ovules in the 
same ovary had already been fertilized. While these nuclei are fusing, the 
three cells that remain in each of the four peripheral groups have taken on 
more or less of the appearance and arrangement of an egg-apparatus. 
In Fig. 19 <2, the group at the bottom of the sac already shows this form, 
and the other groups are assuming it. It will be noted that the cells in the 
group indicated are rather more equal in size and form than is quite typical 
for an egg-apparatus, and this equality is often much more marked (cf. 
PL XXVI, Fig. 25). Another variation in form occasionally seen is that 
of the uppermost group of Fig. 17, where one of the cells is small and the 
two others are elongated, with their nuclei lying at their free ends— the 
whole group appearing to consist of two eggs and one synergid. This is 
exactly the appearance described by Modilewski 1 as typical for the two 
lateral cell-triads of the sac of Euphorbia procera , which is constructed 
similarly to that of the Penaeaceae. In the case of the Penaeaceae, however, 
this or any other deviation from the normal form of an egg-apparatus is shown 
indifferently by any of the groups, and examination of several hundred 
embryo-sacs about the fertilization period has made it clear that these groups 
are as alike in structure as they are in development. Indeed, when (as quite 
often occurs) two groups are equidistant from the apex of the sac (Fig. 21 a) } 
or, as has been several times observed, three groups are thus placed (Fig. 21 b ), 
it is impossible to say before fertilization which is to function as the egg- 
apparatus. For this reason I have preferred to use the non-committal term 
‘ peripheral group ’ for all of these cell-triads. 
In the usual arrangement of these groups (that of Figs. 17-19), naturally 
the one at the apex of the sac has the best chance of being fertilized, and 
from it the embryo therefore usually arises. In several cases, however, 
embryos have been observed developing at the sides of the sac in positions 
so far removed from its apex as to suggest that, if the sac possessed before 
pollination the structure shown in Figs. 17-19, fertilization must have taken 
place in one of the lateral groups instead of that nearest to the apex. The 
clearest case of this— -one in which the remains of the apical group can still 
be seen — is shown in Fig. 29. This also illustrates the only certain case 
of polyembryony noted. As there is no indication of parthenogenesis or 
apogamy, but on the contrary pollen-tubes are freely formed and fertilization 
has been several times observed (though in the apical group only), this might 
perhaps be provisionally interpreted as a case where normal fertilization has 
occurred in two of the other peripheral groups instead of in the apical one* 
This suggestion, however, is only tentative. 
1 Modilewski, 1909, p. 22. 
