CELL-NUCLEUS—FERTILISATION 239 
sexual nature of these two kinds of gametes 
depends, but it seems pretty clear that it is 
connected with the shedding off of some 
substance during the course of their develop- 
ment. | 
~ When a sperm unites with an egg, the 
life history enters on the “‘fern”’ stage. The 
fern plant, like the prothallus, may undergo 
vegetative multiplication in various ways, 
but sooner or later this ‘‘ asexual ”’ generation 
normally culminates in the production of 
spores, just as the prothallial generation 
closes with the production of gametes. 
But the fern does not always go rigidly 
through these stages in a perfectly invariable 
manner. We are acquainted with a number 
of kinds in which the spore-bearing fern leaf 
may grow out directly into a_prothallus. 
Sometimes a prothallus sprouts from a spor- 
angium, and then all the spores die away. 
Furthermore, these prothalli may bear male 
and female sexual organs, and from the egg 
a new fern plant may arise. What has 
become of alternation of generations in such 
a case, and how are meiosis and fertilisation 
respectively affected ? 
Taking the second point first, it may at 
once be said that prothalli formed in this way 
resemble the fern in that their nuclei have not 
undergone reduction. Meiosis has been omitted 
from the life history. But as a consequence 
of this, the egg is already provided, as also 
are the sperms, with a double set of chromo- 
