210 Alternation of Generations and Ontogeny. 
sporangium. 1 Again, the spore-mother-cell and the egg in an 
ordinary homosporous fern are both separate cells, both possessing 
all the characters of the organism ; though technically not both 
germ-cells, they have physiologically a similar relation to the parent 
plant. They develop under similar conditions of protection and 
nourishment, yet their behaviour is very different. The obvious 
view would seem to be that they have impressed upon them in the 
process of development different “tendencies,” in the spore-mother¬ 
cell all characters, except that of tetrad formation, being thereby 
reduced to latency, just as in the development of the ordinary 
plant the leaf-cells have all determinants, except the leaf-ones, 
reduced to latency. 
It would seem also that we have direct evidence that the spore 
and egg are different, for even before the spore leaves the sporan¬ 
gium it has clearly started on a different line of development from 
that of the egg. While both remain protected and supplied with 
nourishment, the spore surrounds itself with a thick two-layered 
wall and stores up a certain amount of reserve material, while the 
walls of the egg remains thin and no material is stored up. The 
two cells have clearly become adapted to start life under different 
environments, and this difference of structure, developed while the 
cells are under similar conditions, cannot easily be explained 
unless we assume internal differences 2 . The differences in what 
are called “ morphogentic powers ” are only separated in degree 
from these differences. 
1 It may be claimed against this case that the microspores and 
megaspores are sexually different, and the sex factor may 
inhibit the developments of secondary sexual (somatic) 
characters in the two cases. The difference between the 
microspores and megaspores, however, is not produced by a 
reduction-division, for the microspore- and megaspore-mother- 
cells are already different, as their products show. The mega¬ 
spore mother-cell has clearly in it latent male characters as 
in development a “ parthenogenetic’’ egg of an angiosperm 
will give rise to an ordinary monoecious plant. Clearly then 
the latency of the male characters has been produced in the 
ordinary process of development apart from the reduction- 
division. If latency of certain characters can thus he pro¬ 
duced in a cell in the ordinary processes of development, 
why not latency of gametophytic and sporophytic in the egg 
and spore ? 
2 A further assumption can, of course, be made that such cells 
as spore-mother cells, young spores and the egg are not 
physiologically separate from the plant, but that their 
development is controlled, not by their own internal factors 
alone, hut by special chemical substances secreted by the 
parent plant. Such an additional hypothesis is elaborate 
and not very probable, and is not borne out by the behaviour 
of animals where, in allied groups, the egg may develop 
similarly, whether enclosed in, or free from, the maternal 
tissue (vide infra). Also it will not explain the different 
behaviour of the free germ-cells of Dictyota, of the Uredinese, 
and of the protozoan life-cycle, 
