THE GENETIC SYSTEM 45 



peared were in pairs, as in Drosophila, they reappear paired 

 as before. Often at disappearance the maternal and paternal 

 sets are in two separate groups (as in figure 16) ; they re- 

 appear in these same groups. Often two particular chromo- 

 somes that are close together or intertwined reappear in 

 this same condition. 



Thus, even in cases in which the chromosomal vesicles 

 are not separately detectable during the "resting stage," 

 it is obvious that each chromosome reproduces itself, in the 

 same form, size, and position. 



II. All this is repeated at every cell division (though 

 the arrangement or relative position of the chromosomes 

 gradually changes in the course of many cell divisions). 

 Thus finally each cell of the body contains the complete set 

 of 2« chromosomes, half from the mother, half from the 

 father. 



D. The Production of Germ Cells, in Relation to 

 the Materials of Heredity. {See figure ly) 



Having followed the main features in the behavior of 

 the chromosomes in forming the body of the individual, 

 we next examine what happens when the individual repro- 

 duces: the processes in forming germ cells and in producing 

 offspring. In dealing with heredity, it is particularly im- 

 portant to have these correctly in mind. 



1. The individual begins as a single cell, the fertilized 

 egg (B, figure 17). 



2. It divides into many cells, some of which produce the 

 body, while others produce the germ cells (C). 



3. At an early stage in development there is usually 

 among the numerous cells one that is later to produce all 

 the germ cells; this is the primordial germ cell (figure 



17, P). 



4. The primordial germ cell in most organisms has the 



