CHROMOSOMES © 257 
species, it follows that either during the formation of 
the gametes, or at some one or other of the cell 
divisions leading up to their formation, there must occur 
a reduction in the number of chromosomes to one-half 
of their former number. In the case of the higher 
animals this reduction takes place during the two cell 
divisions which directly lead up to the formation of the 
gametes themselves. In plants, on the other hand, the 
reduction takes place during the formation of those cells 
which are known as spores. From these, after a certain 
number of intervening cell generations, the gametes 
take their origin. These intervening cell divisions in 
plants are characterized in every case by the appear- 
ance of the reduced number of chromosomes. In 
the higher plants, in fact, a generation is, as it were, 
interposed between the reducing division and the 
actual formation of the gametes. For the spores are 
themselves unicellular reproductive bodies like the 
gametes, but differ from the latter in the fact that 
they develop without undergoing conjugation, and 
give rise to a larger or smaller mass of tissue consisting 
of cells with the reduced number of chromosomes. 
From the fact that the cells of this gamete-bearing 
generation contain half as many chromosomes as 
those of the spore-bearing generation with which it 
alternates, the generation produced from the spores 
has been spoken of as the x-generation in contrast 
with the ordinary, or 2x-, generation. In animals the 
x-generation is reduced to a single generation of cells 
only, which is represented by the gametes themselves. 
We must next proceed to examine the actual 
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