Discussion on “ Alternation of Generations .” 105 
to an individual. These cells are the spore and the fertilised egg. 
They differ in one having twice the number of chromosomes that 
the other has, and this difference continues in the cells of the 
resulting individuals. What do we understand by the specific cell 
here ? The individuals resulting from the spore and zygote in all 
Bryophyta and Pteridophyta are very different from one another, 
and we are brought against the problem why two germ-cells of the 
same organism should give rise to such unlike individuals as e.g. a 
fern-prothallus and a fern-plant. 
We may in the first instance assume, as has usually been done, 
that the difference in the spore and zygote is the cause of the difference 
in the products of their development; that the haploid and diploid 
cells are in such different conditions that they necessarily give rise to 
unlike individuals. This is a quite possible view and fits with the 
general fact of the coincidence of the alternation with the cell- 
changes in the normal life-history of archegoniate plants. This 
view is, however, open to serious objections. In the first place it 
complicates the problem of the transmission of characters, for we 
are practically assuming two specific cells for the two individuals of 
the same organism. Each of these cells would have its own set of 
determinants and would carry the unused set of determinants for 
the other generation. It is not clear why this should result from the 
union of two equivalent chromatin-masses in fertilization. There 
are also facts which directly oppose the view that the cell-difference 
in the cause of the difference between the two generations. On the 
one hand we may have, as in Dictyota, the cell-difference without 
differences in the form and general organisation of the two indi¬ 
viduals. On the other hand in certain deviations from the normal 
life-histories of ferns we find the morphological features of the 
prothallus and plant manifested without the cell-difference, which 
this view assumes to be causal. 
We are thus, I think, led to seek for some other explanation of 
the difference between the two generations in archegoniate plants. 
Let us assume, as the facts as yet in our possession appear to justify 
us in doing, that the cell-difference is not the cause of the difference 
in the resulting individuals; that the specific cell of the organism 
whether haploid or diploid, has the same morphogenetic powers. 
Can we find a cause for the differences in the two individuals in the 
life-cycle which appear at first sight to contradict this assumption ? 
I think we can explain the difference as due to different environ¬ 
mental conditions acting on equivalent germ-cells. The facts appear 
to give direct support to this view, for when the haploid and diploid 
germ-cells are exposed to the same conditions, they develop, in 
Dictyota and Polysiphonia , into similarly organized individuals, and 
in the Archegoniatae, where the resulting individuals are so different, 
we find that the spore and zygote have been exposed to profoundly 
different conditions. Let me make this clear, for it is the chief 
point I have to contribute to this discussion. The spore is exposed, 
in germinating free on the soil, to a number of conditions such as 
moisture, air, food-salts, light, early photosynthesis, &c. The 
fertilised egg germinates not free, but enclosed within the actively 
living tissues of the parent individual. It is thus removed from all 
the influences acting on the spore and exposed to a new set of 
conditions in the shape of nutritive and correlative influences 
