STRUCTURE OF THE GERM PLASM 7 



The small particles into which the genie materials are con- 

 centrated — that is, the genes — are strung up in series, like 

 beads on a string. All those that come from the father form 

 one series or string; all those from the mother form another 

 series, another string of beads. Thus there are in the cell two 

 strings of genes. These constitute what I am calling the 

 Genetic System. This constitution of the genetic system is 

 shown as it appears under the microscope in Figure i, while 

 Figure 2 shows a diagram of its chief features. 



The system is made a little more complex, but in a rela- 

 tively unimportant way, by the fact that each string, paternal 

 or maternal, is broken into a number of segments, which in 

 their condensed conditions are commonly called chromo- 

 somes (D, Figure i). Essentially, however, there is one string 

 of genes from the mother, one from the father, so that for the 

 present we may neglect the segments into which each is 

 broken. The different genes in either of the two strings are 

 diverse organic chemicals, with different properties, different 

 effects on the development. 



Each gene, each different kind of material, has its regular 

 place in the string. Thus the different genes can be named or 

 numbered. Gene No. i has a certain definite effect on devel- 

 opment ; gene No. 40 another effect; gene No. 200 another, and 

 so on for all of them. Thus maps showing the relative posi- 

 tions of the different genes can be made; they have been made 

 for certain organisms that have been thoroughly studied. 

 Changing a single one of the genes — altering it, or substitut- 

 ing for it a different one — changes the course of development, 

 and causes the adult organism to have changed characteristics. 

 Altering one of the genes changes the color of the eye ; alter- 

 ing another changes the size or stature; altering another 

 changes the structure of a limb; altering another changes 

 certain physiological processes. Every feature of the organism, 



