CELL-NUCLEUSFERTILISATION 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- 



