300 Sir as burger . — The Periodic Reduction of 
suiting number is quite indeterminate. Thus, in the lower 
nucleus in the embryo-sac of the Lilies, from twelve to 
twenty-four chromosomes make their appearance in the pro- 
phase. The lower nucleus is larger than the upper one, 
though in other respects similar ; and it may be that the 
increase in number of the chromosomes in the lower nucleus 
is a definite result of more ample nutrition, and the same 
influences may be at work in those cases of apogamy in 
which the number of chromosomes characteristic of the 
asexual generation is attained in a purely asexual manner. 
The case of the adventitious development of embryos in 
Phanerogams is not one that offers any difficulty : for the 
cells of the nucellar tissue from which the embryos spring 
already contain as many chromosomes as does the fertilized 
ovum. But the case of Fern-prothallia from which the cor- 
mophytic asexual generation is developed as a bud, is al- 
together different. The nuclei of the prothallial cells contain 
only half as many chromosomes as do the cells of the asexual 
generation : hence it is probable that on the development of 
the growing-points of the asexual generation, the number 
of the chromosomes in the nuclei is doubled. Overton, who 
has already dealt with this problem from the theoretical point 
of view, is of opinion that it presents no greater difficulty 
than does parthenogenesis, and he draws attention to the 
fact that the lower nucleus in the embryo-sac of Lilium 
changes the number of its chromosomes quite independently 1 . 
Direct observation alone can decide whether the number of 
chromosomes in the nuclei of an apogamously developed 
Fern is increased independently ; or whether, though I do 
not regard the suggestion as probable, its nuclei have the 
same number of chromosomes as those of the prothallium. 
If the latter be the case, then the development of the spores 
of these plants is not attended with a reduction in the number 
of the chromosomes. The assumption that a doubling of the 
number of the chromosomes takes place, under the influence 
of correlative processes, in the apogamous development of 
1 Loc. cit., pp. 14, 15, of the separate copy. 
