304 CONCLUDING CHAPTER 
The intimate connection between Mendelism and 
cytology rests to a large extent upon the close parallel 
which exists between the behaviour of allelomorphic 
characters on the one hand and that of chromosomes 
on the other. 
In the germ-cells of the higher animals the allelo- 
morphs of the Mendelian become segregated, being 
reunited in fertilization, and, as a consequence, the 
cells of the zygote contain twice as many of these 
factors as do the gametes or germ-cells themselves. 
Similarly, in the cell processes upon which the vital 
functions of the higher animals are founded, the 
number of chromosomes characteristic of somatic or 
zygotic cells becomes halved at the formation of the 
gametes, the double number being restored by the 
association of chromosomes derived from two separate 
gametes in the process of fertilization. We have said 
that in the higher animals the gametes are sometimes 
spoken of as constituting an ‘x-’ generation, which 
alternates with the ‘2x-’ generation represented by 
the zygote. We may justify the use of these expressions 
by a brief comparative statement of the facts relating 
to the two so-called generations which recur in the 
life-history of certain families of plants. In doing so 
we shall begin our account with the most primitive 
and simplest forms, and then pass on to other types 
which are regarded as standing on higher planes of 
evolution. 
What are probably some of the most primitive 
members of the vegetable kingdom belong to the 
class of the green alge. This group includes a great 
