f\ 



248 GENETICS 



male is made up of several tubules; that within these 

 tubules may eventually be found the sperm cells with 

 plenty of room to move about; that within a single 

 sperm cell is the nucleus ; that after half of the contents 

 of the nucleus has been disposed of there remain four 

 chromosomes ; that within each chromosome beyond the 

 range of vision there are hundreds of genes and that it 

 has been possible in a single chromosome to determine 

 not only the relative arrangement of over thirty genes 

 but also to find out the relative distance between these 



Fewle Male S enes > * wil1 be realized 



that the analysis of the 



germplasm has gone a 

 long way. 



In Figure 76, taken from 



FIG. 77. The chromosomes of Sharp's "Introduction to 



Drosophila melanogaster. 



After Bridges. Cytology," are repre- 



sented the four chromo- 

 some maps of Drosophila corrected to November, 1920. 

 The four visible chromosomes of Drosophila corre- 

 spond to the four linkage groups of characters obtained 

 by experimental breeding and it is a striking fact that 

 no character has yet appeared that cannot be assigned 

 to one of these four linkage groups. The relative 

 length of the four "maps," which has been determined 

 from the carefully worked-over data acquired by years 

 of riotous breeding for cross-overs, agrees remarkably 

 with the relative differences in the actual size of the 

 chromosomes as measured under the microscope. The 

 four pairs of chromosomes in a male Drosophila mel- 

 anogaster are represented in Figure 77. 



