certain Penaeaceae. 
3 6 7 
The mature embryo-sac, therefore, in all the species studied contains 
four peripheral groups of cells, each group more or less resembling an egg- 
apparatus, and four nuclei fusing to form the primary endosperm nucleus. 
Some interesting exceptions to this normal type may now be noted. 
In six cases it was found that more than four nuclei were fusing to form 
the primary endosperm nucleus. In two of these cases, the total number 
of nuclei in the sac could not be accurately computed ; one of the two 
showed either seven or eight nuclei thus fusing, and only three peripheral 
groups present, two of which were normal, while the composition of the 
third was uncertain. In the remaining four, sixteen nuclei altogether were 
present in each case, and it would seem probable that the sixteen-nucleate 
stage of Fig. 16 had been reached as usual, but that then more than 
the customary four nuclei had been contributed to form the primary endo- 
sperm nucleus. Two of these cases showed five nuclei thus fusing (cf. Fig. 
20 b ), one of the peripheral groups being correspondingly two-celled ; in the 
remaining two there was seen a fusion of six nuclei, an extra nucleus having 
been contributed from two of the groups, each of which consisted of only 
two cells (cf. Fig. 23). Two cases should also be noted in which it seemed 
as if less than sixteen nuclei had been formed in the embryo-sac. One of 
these is shown in Fig. 22. Here the large nucleus at the apex of the sac 
may perhaps be interpreted as one of the four original daughter-nuclei 
which has not undergone division (becoming directly converted into an egg) 
or has only divided once. The latter is the more probable interpretation, 
as while the fusion nucleus seems here to be made up of only three nuclei, 
the other peripheral group figured is not connected with it by the usual 
protoplasmic strands, and does not seem to have contributed to it. If this 
interpretation be correct, the second mitosis has been omitted in this group 
also, as only two cells are present in it. In the other similar case, one 
peripheral group is again represented by a single cell, and probably here 
also the original nucleus of this group has only undergone one division, for 
only the normal four nuclei are fusing to form the definitive nucleus. The 
remaining groups in all these cases are quite normal. 
Endosperm. 
With the few exceptions above recorded, the primary endosperm 
nucleus is always formed by a fusion of four nuclei. Whether a male 
nucleus usually enters into its composition is still uncertain, owing to the 
difficulty of interpreting the fertilization processes in these plants — the 
embryo-sac being small, and the male nuclei showing no differentiation in 
size, form, or staining properties to distinguish them from the other nuclei 
present in it. Fertilization, apparently normal, has been observed in the 
apical group of cells ; figures of it have been reserved till the point mentioned 
has been settled. In Euphorbia proceva , where the male nuclei are easily 
