306 CONCLUDING CHAPTER 
place in sets of four, and their production is preceded by 
a reducing division, so that each spore nucleus contains 
half as many chromosomes as the nuclei of the fern- 
plant—the spores, in fact, represent the initiation of 
the x generation. 
Spores take no part in any process of conjugation. 
They at once germinate and enter on an embryonic 
development of their own, giving rise to a considerable 
mass of cells, all of which contain the reduced number 
of chromosomes. Thus in the case of the fern we have 
a small but well-developed x-generation alternating 
with a much larger 2x-generation. The mass of 
cellular tissue making up the x-generation has been 
named the prothallus. 
Certain cells of the prothallus develop, without 
change in the number of their chromosomes, into the 
gametes. These are differentiated in the usual way 
into male and female—ova and spermatozoids respec- 
tively. 
Fertilization of the ovum by the spermatozoid gives 
rise to a zygote in which the double number of chromo- 
somes is restored. In this way the 2x-generation or 
fern plant is initiated, and by the usual processes of 
cell multiplication and differentiation this body 
becomes completed, developing its characteristic fronds 
and so forth. Thus in the ferns the 2%-generation has 
arrived at a high degree of development, and represents 
the chief bulk of the plant. The x-generation, however, 
still embodies a considerable mass of cells. 
Turning to the higher plants, among which we may 
include those which produce typical flowers with 
