306 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 ' %- 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 reur 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 algae. This group includes a great 



